I am working on a project to build a remotely controlled underwater robot. The max depth the robot has to dive is 40cm. I tested the 2.4GHz RC transmitter and receiver and it could work only up to 15cm below water surface, even though not very stable. I googled a lot, but didn't find any proper transmitter and receiver set with minimum 4 channels, in frequency range of less than 100MHz or at 433MHz. Would anyone please suggest where I can find one or how I can assemble one with off-the-shelf modules?
Is the robot going to have an over-the-water antenna?
If not, then electro-magnetic communication is a no-chance under-water, if the water is salty (seas, oceans) or if the distances are long. The underwater communication is usually done with sound (acoustic waves) - which can travel huge distances, thanks to the higher density of water, compared to air.
Here you can find some more information.
More to-the-point information here:
At 1 megahertz the skin effect depth in wet soil is about 5.0 m, in seawater it's about 0.25 m.
It means that in sea water the EM waves will travel no more than 25cm, at 1MHz. The higher the frequency, the shallower the depth.
More formulas here.
Why EM is not the way: Radio-controlled_submarine, unless you have an aerial antenna for the submarine.
If the water is fresh (no dissolved salts) and if the distances are (relatively) short, then the project might have a chance, depending on the specific details.