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If it's going to be submerged most or all of the time, any type of epoxy based coating should work. You might check out board applications.
It depends on the water you are measuring.If you are measuring just plain water almost any conformal coating can be used .
You need to be sure that the power and other leads to the board are correctly sealed is a must. Water can be wicked along the insulation and coating.
This depends on how much eat your board will dissipate.
Anyway, if it's something ultra low power: silicone, several coats of varnish,
hot glue and anything else that sticks, I guess, will protect your board.
My favorite psot here. Its very impressive and helpful.
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