the rear dive planes can be given a little amount of up when you are about 1/3 dive, as this keeps the nose from going to deep and leaving the rear end to cavitate...so it's down, down, up, down...then you are submeraged and your speed keeps you at that basic depth....
why not try a depth control?
and i would be taking that sub to more open water....thats what i do...it is an incredible model, very effective.
I run mine in the might St Lawrence River ( I love when she is in the big waves) by the dam and in the canal and local deep ponds....she is a beauty...no leaks and very very reliable.