For the watergun I was mainly referring to the point that it sent you in the opposite direction you fire when charged.
For me comparing the two Up-Airs that was me comparing the two in knockback. I'll try to check out the weak hits though, I would only see there being the initial strong hitbox and the one where the tail is falling back and a weaker hitbox. Can you explain some of them?
Fully charged water gun only gives you momentum in the opposite direction if you leave the analog in the neutral or in the direction that Squirtle's back is facing. If you hold the analog in the same direction Squirtle is facing, you'll just lose most of your momentum, if not cancel it out entirely. I have yet to integrate fully charged water gun into my game. I haven't found too much of a use for it, though I'm sure it has a place, like in recovery maybe. But even then a quick WD could give you the same effect and you'd keep the charged water gun. "Idk, ramble, ramble, ramble."
Yes, the two up airs are similar strong hitboxes. Squirtle's up air has its hitboxes arranged like C. Falcon's or Mario's, in that it's an arc of hitboxes, each with different knock back. However, like Falco's up air, each hotbox has the same trajectory: up. I've found that there are strong hitboxes at the beginning of the animation all the way until the tail reaches about a 90 degree angle above Squirtle's head, then there are a few (I would guess three) weak hitboxes, having less knock back the farther back the tail falls.