We're not going to exact accuracy here. It's just the idea. I personally like the Megalodon.

Dienosuchus, perhaps?
Ancient relative to alligators, so perhaps it has some sort of water connection?

On the weapons subject. Someone said something about a long range, sniper, thing? Why not the Plesio? It long necked, so that makes it rather ideal to me.

but what kind of ammo will it use?

If we do go with a sniper mode, I agree that Plesio makes sense for it.

As for ammo. I mean... Okay. I know I said Ice AND Water seems a bit much. However. There's one PRIMARY elemental mode for each form, but wasn't the idea that all of the modes would have at least undertones of that element in them? The ammo could just be ice-based across the board.
As I understand, the reason Megalodon is proposed as a Gatling weapon is because of the rows of teeth. So that, as a mid-range weapon attachment, would fire rapid rows of ice teeth. The sniper mode would fire long range ice bullets, and the third mode could be short range shotgun-like ice blasts. The fourth mode could be a water cannon of sorts.

In battle, it could play out like this:
-He transforms into Aqua, then switches to the cannon mode which allows him to suck up and store water into the universal gun weapon.
-From there, he gives the monster a good drench before switching into one of the three range options (short, mid, or long) depending on the situation.
-While they're still soaked, he combats them with his weaponry until they're frozen solid.
-Once they're trapped in ice, he switches to the finisher which combines all of the attachments onto the gun weapon at once and he blasts them to oblivion.

As much as I like the harpoon idea, I think the three different range options might work better conceptually for the form as a whole. It's an idea. ;)

Why is there three, shouldn't there be four? I like the 3 range modes, but we need to mix it up like gatling, harpoon, etc. We could do short range, long range, harpoon, and gatling.

Maybe we can combine the harpoon and the close range into a bayonet-like attachment that can be used for melee , fire like a harpoon or be used like a grappling hook.

Now if there's one thing missing its a burst/ grenade launcher like mode, maybe one that fires compressed water that upon hitting, releases the pressure and blows up like a sack of tnt

I think the water cannons could have that function as well I mean you think shoulder mounted cannons and I thing explosions...Kalish-splosions but explosions all the same

just have the hisatsu be all his cannons attached to his body and he just barrages the monster to nothing.

Wait are we going with handheld multi-mode guns of body attached guns. I'm fine with either but I'd like to hear what you guys think