Are your motors carbs or fuel injection? If carbs always run the motors dry and drain the carb , really easy to do, if you are not going to use the boat for a few weeks or longer.
Do you have a water separator before the engine? do you have one on the engine, many do and people dont know.
To tell you the truth I dont treat my gas at all. I also run my boat at least once a month. I try to use up as much gas as I can between fill ups. In other words if I pop out for a short fishing trip and only burn 10 gallons out of 100 I don't run to the gas station to fill up the missing ten. I general don't re fill unless one tank or the other is down to 1/4 or less. I believe, regardless of facts :wink: , that this keeps the tank cleaner and does not allow deposits and water to collect. I don't think that water and other deposits are the act of one fill up or another, but the accumulation of many tanks of fuel and the deposits that cone with them. I also don't believe that a large amount of fuel in a tank is going to go bad this year. Most if not all fuel /motor problems are caused by the small amounts of fuel in your lines and carbs. So why do they treat the gas in your tank? Well that's how it get's to you carb's don't it?? I would rather run a injector cleaner in my fuel to keep the injectors/carbs clean then worry about the fuel in the tank. but hey that's just me.
p.s. I also like to run a de-winterize or fog off spray thru the carbs once a year. It really helps keep your carbs working partrs clean and removes any carbon build up in the valves or pistons. Great for two or four strokes.