RE: Lets hear about your Thunder Tiger Neptune SB-1.
I have also had good luck with my RTR sub to depths of fifteen feet in my local, clean, spring-fed lake. I used a line attached to sub and a sliding bobber to measure the depth. Still had control. The antenna is unchanged from the factory configuration, wrapped through the holes around the edge of the forward bulkhead, forming a ring.
Same sub acted badly in my pool, however: servo jitters, loss of throttle control and finally complete signal loss, all at a depth of three feet!
Why? Because while it is a fresh water pool, there is enough salt in it for the chlorine generator to function and that significantly absorbs the radio signal! It does not need to be a 'salt-water' pool in the sense of being the same salinity as sea water. Even if your pool uses NO salt it can have a very high TDS (total dissolved solids) that can quench the signal, especially if it is a concrete pool. You local pool supply store can test that for you.
See if there is another body of water you can test it in (with a tether attached!)
I hope it is as simple as that for you.
Paul