The RX manufacturers worked out what they thought was the best length. Shorten it at your own peril. The total length should be the same after fitting the whip or whatever else is used. A length of semi-rigid plastic tube can be used to support the aerial wire above deck, run the wire up the inside of the tube, fold the end of the wire over to stop it dropping back down.
The current best answer is to go 2.4GHz.
Much depends on the type of boat - a small plastic scale one that almost vanishes at over 50 yards is OK with the wire coiled, similar but larger, run the wire round the deck edge, yachts have rigging to run up.