As a rule of thumb you can say that the lower the frequency the better it penetrates the water. However. I'm not sure if this applies to frequencies in this range. The effect might not be noticable.
Do however keep the legislation of your country in your mind.
Here in the Netherlands it is (since 1999) illegal to use the 35Mhz Transmitters for anything else than flying.
It is kind of a big issue here since they allowed it in say 1989 and banned it in 1999. There are a lot of modelshops that don't know about it and they still sell 35Mhz units to boatbuilders and carguys.
Another thing you might want to check out is if babyphones (intercoms) are in the same frequency spectrum as your transmitter. 27Mhz is used here in the Netherlands for such devices and there are channels that use the same frequency as model transmitters. You wouldn't want to have to dive for your model when the neighbours baby starts crying would you.
Last thing is investigate which frequensies are most common in your area and don't use those.
For pro's and cons of dual conversion check Google.
One last piece of advice.
Don't use PCM transmitters and recievers as they shut down the reception at the first sign of trouble. They are also know to have problems with some addon gear like x-mixers and stuff like that.
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you get older because you stop playing.