Squirtle and Wartortle both have squirrel-like tails, and Wartortle has those furry ears.
Blastoise just gets rid of both of these traits in favor of cannons coming out of its shell.
It's a really plain and boring design, and the cannons don't make any sense.
It's funny, because I remember that sometime during Gen 4 I looked back at Blastoise and thought, "huh, those cannons are really out of place in Pokemon." I don't think a newer design could get away with something crazy like that anymore. Goes to show you how we much we used to take Pokemon designs at face value...