There's one of these for both the other pokemon, I figured Squirtle needed one too.
Pope, great job making sure none of the Pokemon feel left out. <3
Considering how much Squirtle love is usually on this forum, I can't believe this isn't practically over-running with posts on how cute and adorable he is and how that will win you the match.
Anyway, Pope's absolutely correct in the first post, but I would refrain from the second one too much, especially since it can lead to Squirtle's number one problem: getting grabbed.
Anyway, here are some other approach options:
Shell shifted Dtilt- The pop up is pretty significant at higher percents, and can lead to you getting a Fair or whatever you want to do in the air.
I've also realized that you can do ANYTHING out of shell-shift, including Utilt. Nice versus Spacies and heavy alternates.
By the way, you can RAR out of shell-shift, too. I think its probably the hottest thing I've ever seen, and is the primary way I aproach (With Bair).
As far as the Dair someone was describing, it provides some pretty good shield pressure if you jump back and forth between an opponent. It'll probably shield poke, eventually.
And finally, Nair is a great move, especially since it has multiple hit boxes. Method of choice to remove Pikmin effectively.