Yay :] also not to bring race into this but I honestly was drawn to this avatar like a lot...it's probably because of my preference and how I see Bowser...but let's not forget that I'm black <.<
ANYWAY, BACK TO MATCHUPS:
Squirtle: 55-45 Bowser or 50-50, take your pick. I don't think it can really be in squirtle's favor but I saw some low tier tournament match on youtube where Vex played a PT at HOBO, and I cried at the hydro-grab gimps or whatever they are. That **** is dangerous. But generally speaking, minus the gimps and grabby stuff Squirtle can do to us, let's not forget what Bowser can do to Squirtle off of grabs. Since Squirtle is small and quick, it can be a pain to get a grab on him (I should be calling squirtle an it but F pokemon rules right now). However, when you do get a grab, as with any matchup that doesn't involve DK, make the most of it. We can air release Squirtle, and more importantly, we can ground release into a lot of things.
On the downside for Bowser, he's fat. Squirtle isn't. Squirtle is very good, and has quick aerials which translates into FFFFF for Bowser. I can't really see myself get hit by an uptilt from squirtle but hey, you never know. Anyway, uptilt is a very good move and squirtle can usually get about 3 or 4 because Bowser is fat, and then he can get you into the air and have you at his mercy, or lack thereof.
Killing wise, it's our advantage because squirtle is very light and a ground release DTilt should send him on his way. Even a fresh ftilt and a bunch of other "fresh" moves in our arsenal could get the job done.
I'd lean towards 55-45 Bowser because there's no escaping the fact that Squirtle is definitely a legit character. However, Bowser has things that can hurt Squirtle badly, namely our Grab releases, and our upB OoS (since mind you all of squirtle's attacks involve it getting within close range of Bowser, squirtle has no projectile that damages/ranged moves)
Also we could look into how simple it is to gimp Squirtle since his recovery involves him going upwards. Removing his second jump and getting the edgeguard should be good enough, right? The same problem applies to Bowser since his recovery doesn't involve a cypher or a shuttle loop, but Squirtle's isn't great either haha.
I'll get to Ivysaur and Charizard after classes, but I think the overall Matchup is maybe 55-45 Bowser's favor, possibly 50-50 but I doubt it.