The watch is thrown into the pool twice.
When the helicopter lands on the sub and then gets blown up, if you look closely, you can see a string pulling the toy sub.
When Bart Fargo comes through the trap door and flips the two henchmen who are seated, both men lean forward simultaneously in anticipation of the stunt and then stand and throw themselves off their chairs.
At the end when Fargo's watch falls into the pool, we actually see a crew member's hand holding the watch for several seconds before dropping it into the water.