There are many problems that can go wrong with a radio, its just a matter of checking a few things to see whats going wrong.

Signal Problems
This is one of the most common problems, the question is always asked 'why am I not getting the distance?'. First, check that you are the only person on the frequency you're using. if you're absolutely sure this is not the problem read on.

Make sure the aerial has not been chopped, striped back or extended. The manufacturer makes it a specific length for a reason so it should stay the same. You can however make sure that it is as high up in the air as possible and not tangled up in the boat somewhere amongst other wires.

Make sure the battery is properly charged up for the receiver. Try not to run the receiver through BEC circuits on speed controllers. Check batteries in the transmitter.

Make sure that all connections are secure on the receiver. Also tighten, slacken and tighten again the aerial on the transmitter (if possible).

Ensure the frequency that is in the transmitter is within the range the manufacturer made the transmitter for. Although it may be the right frequency there is usually a point where the transmitter doesn't work. E.g. Futaba radio's usually go between 40.665MHz and 40.705MHz. Hitec will not accept these frequencies.

Sometimes the potentiometers get dirty inside the transmitter causing the response to become jittery. Try to clean the pots using contact cleaner. You will need to remove the transmitter covers to do this properly.

Servo Twitching
I have drawn up this flow chart to solve (or at least try to solve) servo twitches.