There are no specific rules for what happens if knocked prone while swimming, therefore, from a purely mechanical standpoint, we must assume that the general rule applies.

From PHB 292...

• A prone creature's only movement option is to crawl, unless it stands up and thereby ends the condition. (Note: Crawling costs 2 feet of movement per foot moved. Standing up costs half your speed. See PHB 191)

• The creature has disadvantage on attack rolls

• An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage

So, how to explain this in terms of being underwater?

This is what I have done in my games with two different parties, and it has worked without complaint from my players.

So, what would be roughly equivalent to 'Prone' while in the water? Being knocked off balance, or knocked into something like a tumble. (In this case, you did just get run over by a huge fish...if you get rammed with force that would normally bowl you over while swimming, you tumble instead)

You will have a harder time moving the way you want to unless you put forth some effort to stabilize yourself.

So, you can 'Crawl' through the water in the sense that you can still make some headway while tumbling, but not nearly as fast as you could normally swim. If you expend half your movement, you stabilize yourself ("Stand Up"), and go back to normal swimming.

You have Disadvantage on attack rolls because, again, you're kind of flailing about in the water.

People close to you have Advantage to hit you because you can't really defend yourself like this...but people at range are going to have Disadvantage to hit you because you're tumbling/flailing around, not presenting a consistent profile.

This doesn't make perfect sense from a physics based standpoint (fluid resistance would arrest your tumble, given time...and you'd drift position as you tumbled), but D&D is not a physics simulator.

Is it overpowered? As you've written it, perhaps.. but only because you worded one of your abilities to always knock something prone, this is problematic.

If a character passes a save you can't and shouldn't knock them prone. This is bad design as a save should let the character in question avoid the worst parts of an effect. In this instance, the lesser of two evils (this being the knockback effect) would likely be applied, perhaps in a smaller denomination compared to a failure. Perhaps something along the lines of "Prone and 20ft knockback on a failure" and "10ft knockback on a success". This is fair enough for your PCs to come up with some creative uses for it (ship combat comes to mind) and if its turned against them they'll likely still feel that it's fair enough (aka I'm further away but at least I'm not prone.)

Generally, to determine if something is overpowered, we have to gauge it against something else already that already exists within the scope of the game and evaluate the merits based on something similar.

So currently you have:

• An effect that knocks a character backwards and prone on a failed save.
• An effect that gives characters using ranged attacks disadvantage.
• An effect that creates an area that gives things inside it disadvantage on perception rolls.
• An effect that makes ships move faster due to controlling wind.

Currently the closest spells to emulate these effects are:

• Grease + Gust of Wind ( A first level conjuration spell, a second level evocation spell )
• Blur ( A second level illusion spell)
• Fog Cloud ( A first level conjuration spell )
• Control Wind ( A fifth level transmutation )

These are all things a ninth level wizard would be able to do without any trouble with better effectiveness. If you removed the last effect, A third level wizard would be able to duplicate the effectiveness of this weapon.

If you want it to be more balanced. Amp it up some but remove the auto-prone. Give the weapon an enchantment bonus, and ten charges that refresh daily after a long-rest. Make the first level effects cost one charge, the second level effect cost two charges, and the fifth level effect cost five charges to limit how much you can do with it per day.

Also, Since the DC12 is a pretty low and very easy save to beat at tenth level for most things, you could make it so that for every three charges you use you could increase the save DC of the weapon's prone / knockback effect by one, which would bring the save up to around DC15 if you were in a pinch and needed to burn the charges to get something away from someone / knock someone down.

As a balancing mechanism you could even go so far as to have the weapon lose its enchantment bonus for the day if all of its charges are consumed.

So as far as the weapon goes, here's a general Stat block:

Kusanagi no Tsurugi

Weapon(longsword +1), Very Rare 10/10 charges

This yellow bladed saber with a green hilt can control the wind, sending a gush of strong wind in the direction of the strike of the sword. Kusanagi has a fine edge, tempered in the wind, and as long as the weapon is imbued with magical strength, it retains its +1 Enhancement bonus. When all of Kusanagi's charges are used, the weapon becomes a non-magical longsword until the user takes a long-rest in a place where wind is present.

As an action, when drawing this weapon from the sheath, a slash of this sword sends a 60 foot cone of wind in any desired direction. Any creature inside this cone must make a DC 12 strength saving throw being pushed back 20 feet and falling prone on a failed save or being pushed back 10 feet on a successful save. This pushes fire out of the cone's area unless it hits a wall, in which case it snuffs out. It also disperses fog. Air elementals and other non-corporeal creatures have disadvantage on this saving throw. (1 charge, +1DC/3 charges)

Kusanagi no Tsurugi can also create the following magical effects, provided the weapon has charges:

• It can be used as a reaction to give disadvantage on an attack roll with a ranged or thrown weapon attack. (2 charges)
• It can blow sand, dirt or snow in a creature's direction in a 30-foot cone that persists for a number of rounds based on the charges used. All creature inside the affected area have disadvantage on Wisdom(Perception) checks that involve sight and a disadvantage on their attack rolls. The user must be in a place with sand, dirt, or snow to use this ability and only one such cloud can be in effect at any time (1 charge / two rounds)
• When the blade is held aloft and the power of the wind the blade possesses is channeled It can increase the speed of a sailing ship by 2 miles per hour by channeling nearby gusts of wind to lift the sails. This power requires concentration. (2 charges)

Kusanagi no Tsurugi is a blade made of and forged by the wind, in places where no wind is present such as deep underground or underwater, the charges of the weapon cannot be used.