How's your skill level at dismantling r/c stuff?
You have to get at the circuit board located within the sub. I've had this problem before with the newer Nikko Seawolf (black) . It took the sub apart to make adjustments to the trim pots. There are three (3) of these located on the circuit board. One is for the Dive motor , another for the steering (left & right) and the last for forward & reverse of the propusion motor.
You have to use a fine flat screwdriver. A plastic or one with a insulated handle is preferable. You have to fine tune or syncronize the sensitivity/response for the proper proprtional output to the three motors these trim pots control.