Paul
40MHz is a VHF (Very High Frequency) set. This frequency band is allocated exclusively for ground-based models (including boats). There are 34 separate frequencies available within this band. These are selected by fitting a pair of the apporiate crystals into the Tx and Rx. One Tx can interefere with another Rx on the same frequency so it's important to maintain some discipline. To enable others to identify which frequency you are using, you must fly a green flag from the aerial of your Tx showing the last three digits of the frequency e.g. for 40.695 it would show 695. In Europe there are only 19 frequencies available in this band and these are allocated numbers from 50 to 89 **LINK** VHF signals will pass through water so 40MHz is suitable for model submarines.
2.4GHz is a microwave frequency set. There are 80 channels available within this band and the choice is automatically made by the Tx and Rx as soon as they are powered up. The set looks for and locks onto the first available frequency and no other Tx can interfere with the Rx. As such flags are not required – which is as well because the Tx aerial for 2.4GHz is a very short stubby little affair. 2.4GHz is pretty much a universal frequency and many domestic gizmos use it, as well as model aircraft and helipcopters. These sets are manufactured largely for small RTF model aircraft and so the weight of the receiver is less than a corresponding VHF radio. The frequency of the radio is irrelevant to the operation of the servos, speed controllers etc, so any servo will operate on either frequency band. Microwaves will not pass through water so 2.4GHz is not suitable for submarines.
The range of 40MHz sets is out-of-sight for model boats whereas some of the cheaper 2.4GHz sets have a range limited by the power output of the Tx and typically 100 metres. The "brand" 2.4 sets such as Futaba, Spektrum, JR etc will have the same range as their VHF counterparts unless stated otherwise – always check first!
We have to accept that there are likely 1000 aeromodellers for every boat modeller in the world, so the RC manufacturers aim their products clearly at the larger market. This means that the radio sets may have special functions dedicated to either fixed wing aircraft or helicopters. Such phrases as "crow-braking" and "idle-up" come to mind. These are practically useless for model; boats and IMHO only serve to confuse or irritate. Also beware when buying a multi-channel 2.4 set. You may find that channels 5 and 6 are not true proportional channels but simple On or Off switches. These can only operate a servo to either one end or the other of its travel, or switch on a simple load circuit.
The Robbe-Futaba F series, which are VHF sets, have had all of the aircraft and helicopter functions removed. As far as I am aware they are the only dedicated boat and truck multi-channel radios available. You can add separate modules to the Tx and Rx which will enable you to operate umpteen proportional and switched channels for your boat, although these are by no means budget items. If you want to operate winches, deck lifts, smoke, sounds and half a dozen lighting circuits etc all on the same model then this is the set to go for.
Finally quality: All of the cheaper 2.4GHz sets are made in China, where price is all and quality means little. I have handled several makes and have been unimpreesed with the build quality of all of them. The Spektrum DX5e is the best of these. I have also two Futaba 2.4GHz sets – a 6EXA and a 6J. These are far better made although, of course, they are more expensive.
Hope that covers everything.
Dave M