Hydrograb seems really "gimmicky" (?) to use as a punish since you basically have to have started it before your opponent has even done anything yet. Maybe if you shellshift around the stage all day you could get a few good punishes out of it, but shellshifting like that means there will be times when u can't shield which is kinda dangerous.
By squirtle's grab game, i meant his grab options once he gets a grab. I feel like his only usable throws are fthrow and dthrow which both dont give you anything quaranteed... fthrow u might get a fair with Bad DI, and dthrow u just get dmg. The two becomes kind of a mixup at kill percents since you have to DI fthrow away to avoid the fair followup, and DI downthrow in to avoid instantly dying.
Hydrograb with good spacing is godlike, I really hope you experiment with it some more.
As far as squirtles throw/followup game I have a few tricks, they of course depend on falling speed of the opponent.
Fast fallers low percent, D-throw, run pivot-grab them or just set up a tech chase, or slingjump aerial them. Once they're above 30% I'll upthrow up-tilt them until they DI out then I regrab and f-throw D-throw DI trap until I get them offstage.
For a lot of other characters I can up-throw up-tilt regrab at the lowest percents, then at above mid percents his grab game starts to require advanced techniques to let him followup like reverse hydroplane U-smash, or slingjumping after them.
Honestly as squirtle I feel his movement let's him punish characters who are falling back to the ground (as long as it' isn't jiggs or something), so I'll commonly D-throw to mess and let them DI up and then fling around the stage setting up position to destroy them as soon as they try and land, Up air can beat out an amazing amount of attacks due to his tail being intangible.
edit: double post :s
just to make this post meaningful, i think all Squirtle players should use fsmash more bcuz its hella safe and it eats everything
I don't think it's safe, for instance it's shield grabbed really easily. But I really like hydroplaning it, it's really good. It's also the move I use to rest punish jiggs because up-smash can be immediately shielded after the initial hit (which is a balance to it since it can shoot across the stage with full charge and huge range lol)