Underwater RC System

There are two problems.

1/ RC control units may only transmit at a certain power level as defined by the regulation. You are also restricted to certain frequencies as well.

2/ 27Mhz will not penetrate deeply into water as it gets absorbed quickly. You need to transmit at a much lower frequency say 300khz. However at this frequency the data rate is much much slower. This would lead to much time lag between instruction by the controller and the reciever/servo operating.

You could try to put more length on the submarines antenna to improve the reception.

You could increase the power of the transmitter by using a RF linear amplifier. However it may/will cause distress amongst other lake users by swamping their signals. It will also bring you to the attention of the communication regulators.

the RC radio's like the one you have listed are all but extinct. the technology has gone to spread specktrum 2.4 gig. one radio will not inter fear with another one. the distance ability is far Superior to the old Crystal systems. i would contact futaba or specktrum and ask them. i do not think any company would guarantee it,too many variables,like your batteries not being fully charged,or an in properly installed system.the receiver uses two antenna's that are to be set at different angles and distance. I had the DX-7,it was the best radio i ever owned.

It's probably illegal to "use ultra high power FM27 Mhz transmitter ".