The rocking motion of the boat maybe causing a poor wire to disconnect?
What equipment is he using?
How old are the battery packs? (Black wire syndrome)
Do they charge enough to give a signal at at anything other than close range?
Is any water shorting out connections?
Bleed over from other transmitters (already mentioned
)
Failing these things and the things already mentioned I would (and did recently) replace each item in the circuit one at a time (with items from a known working model, even if they are borrowed) don't be tempted to change more than one item at once.
Also, make sure your transmitter is not in low power check mode (futaba have this and I'm sure the others will also)
Try also taking your transmitter crystals and putting them in a known good transmitter to rule out a faulty transmitter.
Finally, let us know when you find it please. Good luck. U2