The further the submarine is from shore, the less depth it can achieve because the radio signal must travel further. Generally fresh water provides better radio transmission than salt water, thus the RC submarine can go deeper in a pool or freshwater lake than in the ocean.
Visibility is also a factor. If you can't see the submarine because it is too deep, too far away, or moving too fast, then you may have trouble knowing when it has moved out of radio range and you may need to start RC submarine recovery steps. Going deep where there is limited visibility also increases the chances of getting snagged by underwater obstructions and not being able to resurface.