STAB gives a 50% boost to moves that are the same type as the user Source
Eruption/Water Spout has 150 BP when the user's HP is full. the formula to calculate this is 150 × CurrentHP ÷ MaxHP. Source
The Choice Band raises the power of physical moves by 50% while locking the opponent into a move. The Choice Specs raise the power of Special moves by 50%. The Choice Scarf raises the speed of the holder by 50%. Source
Water Spout and Eruption are special moves, so the Choice Specs will boost them, not the Choice Band
Type Gems raise the power of a move that is the same type as the gem by 50%, but the gem is then consumed. Source
- Critical Hits always make a move to twice it's regular damage. Source
EDIT: I'm not sure if you want all those added, or if you want to know how to calc them individually, so I'm going to add them anyway, just to cover my bases. If anyone sees any math mistakes I apologize, I used a calculator, but I might have typed the numbers wrong, however I am fairly certain that it is correct.
So, Add all those up and we get:
150 (BP for WS/E) + 75 (STAB) = 225 + 112 (specs/gem boost) = 337 x 2 = 674
The above calculation is assuming both pokemon are unboosted, there is no weather in effect, and that the target is taking it with neutral damage from it.