Full tank is best (although it does feel like you are putting $60-$70 in a mattress these days). Keeps the air space to a minimum to limit condensation.....as long as the appropriate fuel stabilizer is used for long term storage.
Less of an issue for "shorter" storage such as a couple of weeks or days.
Water in the tank could be attributed to a problem with the vent check valve at the waterline on the hull. Air should go out, but water should be able to go back in.
Proper fogging of the engine and carbs will also help prevent corrosion in moist environments. Also, make sure you leave the compartments partially open to ventilate the hull.
