An all spell caster party already has balance built into it.
They can pretty much do whatever they want. Situation arises where they need a lot of meat shields? Summoning spells or animating the dead. A bunch of magical weapons and armor are coming their way and threatening to beat them dead? Dispel and Antimagic Zones. Lost of casters attacking them? Counterspell!
Spell casters are incredibly versatile. Because of this, balance is going to be based on how many spells they're casting per encounter, as well as the availability of wands, rods, scrolls and staves.
After that, just leave it up to them. You may find sending a half dozen orcs at them is a joke and they easily blow them away. So the next group staggers their approach to not get caught by AoE.
Ultimately, they're going to either breeze through situations by expending spells like crazy and saving nothing, or they'll think their way through problems and be conservative. The balance will happen. It just depends on how you run the encounters and whether or not they seek a resting zone when they're depleted, or just getting on towards depleted.
First, it's good if you can find out why the player is playing a non-magic user. You found out it matches his character, which is good, but it might be good to find out what type of gameplay he likes, so you know what to provide and what not to avoid, which might be tricky if you yourself always prefer to player magic-using characters.
Some reasons players might avoid magic-using PC's include:
They don't easily relate to magic powers, and are more interested in non-magic human experiences and situations.
They don't know how the magic system works, and don't want to have to figure out the complexities of it.
They are interested in some other aspect of play (say, detailed combat), and want to focus their skills on that so it will be more interesting and/or their character will be better at fighting.
It's good to know if the reasons are like the above in that there is disinterest in magic, because if so, it may not help to offer them, say, magic items, or a special kind of super power, or even unrealistic martial arts abilities, if they are similar to the things the players don't like about magic, that had them choose to be non-magic users.
In general, I think if a player chooses to be a non-magic fighter type, they will probably appreciate being given interesting combat situations where their character is valuable. Fortunately, you're playing GURPS, which is the best system I know for that. Use tactical combat with a map, and include foes the fencer will be useful against, and/or more (or more resistant) foes than the magic will be able to take care of.
There are other reasons players might choose non-magic-using PCs, which do invite adding magic or magic-like abilities:
A player might have a super-skilled fighter concept, and enjoy meeting a master trainer who can teach them super fighting skills, which may be somewhat magic-like. These can offer not just fighting skills, but skills such as extreme abilities in hiding, jumping, climbing, silent movement, chi powers, possibly magic resistance through will/meditation/chi, and so on. Probably the best source for this sort of thing in GURPS is GURPS Martial Arts.
A player may not want to cast spells, but might like having/using magic items. If there are enough of magic items available that are useful for non-casters, and the groups gives enough of them to the non-fighter PC, then that can give that PC abilities the others don't have.
There may be other types of super powers available in your world that are appropriate to the non-magic-user PC, such as religious-based powers, or psychic powers, or super powers, or whatever. If the player is interested, those can give the player abilities that the magic-users don't have.
In many games, there is so much common magic and magic-like abilities, and they are so useful and/or powerful, that PCs who avoid magic are just going to be much less powerful than those who do. Fortunately, GURPS isn't one of those unless the setting or magic system used makes magic very common and strong. Standard GURPS Magic is quite strong and if most or all of the spells are available and common, there is a lot they can do, but there are still good niches for good fighters.
Another approach can be to have the adversary NPCs notice if/when the players defeat their minions or some situation/puzzle with a conspicuous use of magic. The adversary NPCs may then try to use that knowledge against the party, and come up with ways to challenge the party that counter the magic they know the party has. Something that looks like it can be burned, but can't, to bait and use up the magic of the pyromancer, for example. This can create a situation where the party needs to fight, and the fighters get a chance to shine.
Similarly, you can study the non-magic-user PCs and see what abilities and knowledge they have that are outside the domains of the magic users, and let the PC get chances to notice those. In your case, the sailing thing comes to mind. If he's the only one with seamanship and they're on a ship, he'll have advantages in knowledge, getting around the ship, not getting seasick, and avoiding DX penalties for a rocking deck, etc.
Having studied the PC's abilities, especially when (as you wrote) the player is not as familiar with the game system as you are, you may want to give that player extra information and prompts and ideas throughout the game, based on those abilities, because there are things the PC might think of that the player may not, and this can help the player grow familiar with the opportunities their skills offer.
There may also be places or factions in your campaign setting, who are opposed to magic, or certain types of magic, so that it may be criminal or at least attract attention of ill will if people know you use that magic.
Always use a hex map. This makes combats more interesting for fighters, and results in more situations where fighters are needed and useful, and casters have more difficulties, due to range, obstacles, being engaged by attackers, etc. When you're using all the rules, including range penalties and casting times, it often isn't actually all that superior to be a mage in GURPS, as it may take too long to cast a spell, or the target may be too far away, or there may be obstacles to line of sight, or someone may come grapple you, etc.
Get GURPS Swashbucklers for some realistic & cinematic fencing-related abilities, tricks and equipment, as well as some sailing-related stuff.
Have situations where the characters have several challenges with not a whole lot of time to rest in between. In GURPS Magic, the magic-users use up mana when they cast spells, and so they'll need to use it sparingly, or run out of mana by the time they get to the later situations. Meanwhile, the fighters tend to be able to do more per fatigue point. The party may even realize that it needs to save mana for healing and even to restore fatigue on the fighters, if it is to overcome a series of challenges without running out of mana to cast spells.
There are situations in GURPS that call for fencers, such as multiple attackers, or dangerous foes who attack multiple times per turn and are best parried, or that are best disarmed.
GURPS has the concept of Low Mana and No Mana zones, and Aspected Mana Zones and so on, which can also take the magic users down a notch in some places.
The easiest way to emphasize your casters in the early game, is to throw in lots of relevant skill checks, particularly knowledge or spellcraft checks. Almost any spellcaster is going to have relevant skill modifiers, and it proves their place in the party.
A related option is challenges tailored to make spells useful. A small item you need is on the far side of a chasm? Summon Nature's Ally or Mage Hand can help with that. Having ambient magic, or an arcane trail to be tracked with Detect Magic also calls out your casters.
Also, it's been my experience that Druid is one of the better low level caster classes, with cleric because of free swapping in of Summon Nature's Ally spells, reasonable combat ability (3/4 BAB, d8 HD, not terrible armor, buff spells), and their selections of durable spells, such as Produce Flame and Call Lightning.
For the Archivist play to their skills and their Dark Knowledge ability. Throw in a fair number of the types of enemies Dark Knowledge helps against, and they should feel pretty useful.