To start off, the Water type is my favorite (just look at my Gravatar), so hopefully I'll be able to provide some useful feedback.
Out of the two you have suggested, Clawitzer is definitely the better choice. Clawitzer has a great ability, better Special Attack (which is good for Water-types), and better stats overall.
Blastoise, though, is better than both of them, and with a Mega Evolution, it's pretty amazing. It's a great tank that also packs a pretty powerful punch.
But since you said "maybe another one," I'll take the opportunity to suggest some awesome Water-types.
First of all, there's Empleon. Empleon has amazing typing, as it is a Steel-type that resists Fire-type attacks and a Water-type that resists Grass-type attacks. Its overall base stats are just as good as Blastoise's, too. I recommend running Scald, Ice Beam, Flash Cannon/HP Fire, and Grass Knot for him.
Swampert is another amazing Water-type. Its stats allow it to tank most moves and then hit super hard. And its typing is pretty much the best around. Don't worry about Grass-types, they have plenty of weaknesses. My suggested moveset would be Waterfall, Earthquake, Hammer Arm, and Avalanche.
Kingdra is an epic Pokémon, as it has great stats and is a part-Dragon type. Because of its typing its only weaknesses are Dragon (which it already has coverage for) and Fairy (which it can get coverage for with Flash Cannon). You can go with either a Physical or special set for him, but I recommend this special one: Scald, Dragon Pulse, Ice Beam, and Flash Cannon.