RE: Max practical depth ?
"Max depth should be around 5 feet ( 1.5m) only due to the radio. radio signal penetration through water is difficult at best."
Not true. In relatively clean fresh water, I've seen boats go in excess of 30 feet without losing signal. Read "clean" as free from chemicals more than dirt. Obviously, there are chemicals in pool water so depth is limited there.
"Higher frequencies will work better than lower freq."
Wrong. It's the other way around. Lower frequencies penetrate water better than higher frequencies.
The above refers to standard R/C systems (Futaba, JR, Hitec, Airtronics, etc.) NOT Radio Shack type "toy" radios.
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