[IXRollOutIX]

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.