In my opinion this version would fit more a rock/water: I'll tell you why.



Altough it's true we barely have any water/fighting type Pokemon, while we have a few water/rock (Mainly Kabutops and Omastar), his shell as you mentioned can endure any explosion.

This is similar to Golem's Pokedex description:



" Its boulder-like body is extremely hard. It can easily withstand dynamite blasts without taking damage." [Pokemon Leaf Green]



Also the fact that he's a tank suits more the rock-type Pokemon. Dulcis in fundo imho his body is clearly made of rock, as you said.



Overall, the sprite itself is Amazing as always and anyway a Water/Fighting Pokemon would be interesting for competitive, also because a fighting-type tank would be new.



Sorry for the long comment ^^"