Anyone and everyone who has a predictable recovery (up/side/diagonal).
Unfortunately, though, if all you're looking for is the water gimp, go with Mario instead. FLUDD has better range and stays out longer if I remember right. He has less weaknesses, has a cape, doesn't worry about Stamina, and is a much easier character to learn because you don't have to concentrate on three characters and switching. Don't try to main/secondary PT with the intention of only concentrating on Squirtle, who can be quite tricky in his own right.
HOWEVER, Squirtle's charge time appears to be MUCH quicker, he can MOVE while charging with no hindrance to his air speed, and it is MUCH more discreet (Meaning that you can fully charge any time, anywhere, and your opponent probably will not notice it and thus not take any precautions against it, while Mario stops moving horizontally completely even in mid-air, so charging it while recovering is a no-no, and he gets a huge FLUDD on his back, with LOUD charge sound effects which basically telegraph that he's planning to gimp you in the near future). You also get the advantage that people believe that Squirtle's Water Gun is useless (HA! Far from it), while they usually know too well to beware Mario's. Also, Squirtle can jump offstage to use it because of his better recovery than Mario's if I remember right, although don't quote me on that last one...
But to answer your original question in detail, almost everyone can be gimped that doesn't auto sweetspot from below, or has been pushed far away that they are relying on their Up B to grab the edge on the way down. If someone is stupid enough to use their third jump early, squirt 'em. Even some tethers can be gimped by pushing them far enough out of reach, if you're daring enough to jump off and squirt... Key characters to watch out for are
Ike (both recovery options. You can spray him as he pokes his head up during Aether), other
Squirtles,
Charizard, stupid Olimars who believe it's better to land on the stage (Some actually do this), Samus (If her screw attack doesn't sweetspot. I forget. Otherwise, don't bother.),
Snake (Even if he's smart enough to be too far away to get grabbed, he still isn't safe!
), it forces a stall on a gliding character like MK if I remember right,
Kirby with his Final Cutter,
King Dedede with his Super Dedede Jump,
Bowser, I believe
Diddy Kong doesn't sweetspot the edge with his Barrel Rockets, but I might be wrong,
Donkey Kong, someone correct me if you can't spray a
Fox/Falco/Wolf using his Illusion/Phantasm/Flash, as I'm pretty **** sure you can, and blow them away if they charge up their Fire moves above stage level by accident, Ice Climbers using Squall Hammer, throw one out towards a Jigglypuff for giggles (it won't work. Just do it for fun. =P),
Lucas and Ness during PK thunda,
Luigi (I believe you can even spray him during his Cyclone recovery),
Mario,
Peach if you knock her out of her floating,
SHEIK AND ZELDA are great targets if you hit them during their "warping" animation, as they're pushed back before the warp (Meaning they will come up short 90% of the time! XD), and last of all, do it against
Yoshi, as you can't knock him out of his second jump, anyways.
But seriously, if you're committed to learning ALL 3 PT characters, use PT. If you only want to use Squirtle for his Water Gun, go with Mario instead.