Hi Tall Paul,
Current TX - 2.4GHz - probably surface to 50-75cms on a good day, with a following wind, winning lottery numbers etc.etc. 😊
Or 40 MHZ - 5-8 feet perhaps
Or UHF (459MHz) openLRS - maybe 100cm
Or 868MHz - no idea of depth.....
Best and most active place I know for Submarine help is
Facebook group - "Dive-in to Model Submarines"