As for the movesets, I think water gun is ranked higher than bite because it benefits from STAB bonuses, and thus provides a higher DPS than bite. Same thing with ice cannon: it's ranked low because it outputs significantly less DPS than Blastoise's water secondary.
That said, you're right that bite/ice beam can be very useful depending on the situation. The problem is that a bite/ice beam Blastoise will probably not be as good as an actual ice-type pokemon with ice attacks. I would say if you need a counter for grass, psychic, and dragon pokemon, and you have no better options, then level up this Blastoise. If you have a Lapras with ice attacks, then I would prioritize powering up the Lapras first.