It is correct, that salt water does limit the transmission range, but unless you want to operate your sub at the dead sea, your sub should still be able to get the signal at a depht of 1 meter. You must remember, that the frequenzy on your transmitter have to be as low as possible. In europe we use 40 or sometimes even 27 mHz. Also having a good nice reduction on all your electronics including motors will dramatically improve the depth at which you can get a signal.
And one thing too, transmitters with a frequenzy above 1,2 GHz (not sure of the precise number but 1,2 GHz is rather close) will not penetrate water. The signals will simply be absorbed in less than a centimeter water.
As closer you get to 1,2 GHz the water absorbs more and more, so keep your frequenzy as low as possible.
When talking about salt water, remember a normal swimmingpool has a far higher concentration of salts (Chlor and natrium based) than sea water. The biggest problem in natural ponds or open sea is normally that you can't see you sub when below water. So unless the waters around you are very clear, transmission should not be a problem if you use a proper transmitter and reciever and you do as much electrical noise reduction as possible
Theoreticly the best waters to sail in is demineralised water (water with no minerals or salts). The worst would be highly poluted water.
Hope it helps