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Seriously, though - I agree with your son about keeping the tank full. Unlike our cars & trucks, boat gas tanks are vented & exposed to the atmosphere. As temperature fluctuations occur, air moves in & out of the tank - bringing in humid air. When that humid air cools at night, condensation occurs.
The fuel would need to expand considerably during the day, and contract considerably during the night.
Then we must factor in;
...... just how much gasoline expands/contracts via temperature differentials.
One search suggests: When gasoline rises from 60 to 75 degrees F, for instance, it increases in volume by 1 percent.
......
how long does it take for the fuel to absorb and react to temperature differentials.
..... just how humid or dry the ambient air is
The space above the fuel would then need to move enough air (in/out) to create a pull/push via the fuel tank venting system.
Do the math....... this equates to an extremely small amount of air movement.
Here is an interesting article by David Pascoe.
http://www.yachtsurvey.com/myth_of_condensation_in_fuel_tanks.htm
Read what David has to say about the more common means of water intrusion (deck fill neck caps, no high loop in the vent line, etc.)
IMO, we should be more concerned about the degradation of the fuel regarding the Octane rating.
The ethanol in gas "mobilizes" the water, helping it move through your entire system. So, a partial tank of ethanol fuel is the worst combination. Using marine fuel will help minimize the effect of water in the tank, as they pretty well separate from each other.
As for pumping the gas out - it should be pretty simple, provided you get some kind of pump. I have a pair of these, that I use:
https://www.amazon.com/Moeller-Marine-Products/dp/B07DQYT7V7
This model is a little expensive, but they work great for draining a gas tank and also for draining oil through the dip-stick, when you do an oil change.
That is very similar to the original Tempo Oil Boy...... and they work great for starting a siphon.
Remove your Anti-Siphon valve, temporarily replace it with a standard barbed fitting, and siphon away!
As for additives to neutralize ethanol effects - I don't know of any, and everything I've heard and read says there is no way to make it truly safe for older marine engines.
I would agree! It would be nearly impossible to eliminate the effects of Ethanol by adding chemicals.