I'd use custom items to accomplish this. First is an item to allow your aquatic animal to walk:
Belt of Piscine Endowment
Aura faint transmutation; CL 5th
Slot belt; Price 30,000 gp; Weight 1 lb.
DESCRIPTION
When above water, this belt seems to be made of sharkskin. When below water, it seems to be made of the skin of a furry mammal.
When worn, this item allows aquatic creatures to grow legs and gain a land speed, exactly as the spell fins to feet.
CONSTRUCTION REQUIREMENTS
Craft Wondrous Item, fins to feet; Cost 15,000 gp.
Since all aquatic creatures either have a belt slot or a land speed (at least, all the ones I know of do), this works on all non-walking aquatic creatures. Before you can afford this item, you can use the Fins to Feet spell, which is a level 3 wizard, witch, and druid spell.
For the ability to breathe, there are a few options, depending on how much you want to stick to RAW.
First is a custom magic item of sea steed (18,000 GP). This is intended to allow land-based animal companions to work as sea-based animal companions, but it works just fine either way. This gives the animal the amphibious quality, allowing it to breathe on land as well as under water. This spell doesn't last long enough to make it practical as something to cast regularly before you gain access to the item.
Second is the Waterskin of Woe (80000 gp). This item lets you remove all the water from a creature, and "does not need to breathe, eat, drink or sleep and is immune to non-lethal damage." Since it doesn't specify what you no longer need to breathe, it should work on your shark just fine. The problem with it is that it deals 1 point on Constitution damage per day that it was used once you stop using it, and can only be used for a number of days equal to your Con score. However, there is no limit to the number of times you can use it per day, so you could easily turn it off at night, heal any Con damage with lesser restoration, and turn it back on while the aquatic creature holds its breath.
Third is the Necklace of Adaptation (9000 gp). There are a few problems with this as written. For one, it is a neck slot item, and the magic item slots for animals guidelines say that neither sharks nor octopi have neck slots. However, you might be able to make the argument that you could put it on a belt slot instead, possibly for a higher cost. Next, it specifically talks about creating a "shell of fresh air" around the user, which doesn't really help aquatic creatures. Again, I think the argument could be reasonably made to allow a shell of fresh (or salt, I guess) water to be used instead, if the item is made for aquatic creatures.
Fourth is a custom magic item of life bubble (56,000 gp). This effectively works like water wreathing, but it doesn't specify that it lets a creature breathe air, only that it lets a creature breathe comfortably. It actually creates a 1 inch shell of "tolerable living conditions", which could be fresh or salt water as easily as air. For a shark, it's probably moving water, which will likely be a tad inconvenient.
Since fins to feet is 3rd level, and life bubble is 4th level, you can have a land-based shark or octopus by 7th level for the cost of 4 spells per day. 2 castings of fins to feet will get you through an adventuring day, and 2 castings of life bubble will get you through the 24 hours that you need to keep your animal breathing. Since most aquatic creature have limited equipment slots, you will likely need to put two of these effects on the same item, likely a belt. This will cost the same as the added cost of the two effects, plus half the cost of the cheaper ability (or the new ability, if created in play).
IF you want to go the super-expensive route, you could also gate in a Personification of Fury every morning, since it has a special air breathing spell-like ability, which works like water breathing, but lets aquatic creatures breathe air.
I found a way to unlock the 2 feats gained at HD 1 and 3 for the wolf. A human with the alternate racial trait Eye for Talent can start their animal companion with +2 in any stat. Pop that into Int and you have a 4 Int animal from HD 1, unlocking all feats from the get-go.
My tripping wolf companion now gets the feats, in order:
Dirty Fighting, Improved Trip, Fury's Fall.
After that, I think I will coordinate my character and companion to each have Tandem Trip, but that is still up in the air.
Anyway, to get feat selections earlier, Eye for Talent. You end up trading 1 character feat for 2 feat unlocks on your companion.
Best Answer
I'm not sure if you could raise your animal companion level past your Druid level, but you could always be a Pack Lord Druid and take Boon Companion twice to buff the level of each companion, and have two max level companions until level 9.
If you are an Aasimar, you could take the feat Celestial Servant to give your animal companions the celestial template.
I don't think as it stands there is a way to do this via Druid, but a Lunar Oracle should fit what you're looking for just dandy. Just take the primal companion revelation:
After that, use the favored class bonus of an Aasimar Oracle:
The original version of this (before a July 2015 errata) let you have +1/2 instead of +1/6. Despite that, you can use the spells Righteous Might and Divine Power (which stack and can be cast on your animal companion via the Share Spells feature). Also look into Divine Vessel for a disgusting display of animal companion power.