The problem with gas as I understand it (after poking around on the wild world web) is that (1) the lighter parts of gasoline evaporate during storage leaving a heavier less potent fuel, (2) gas breaks down during storage via oxidation, (3) ethanol in fuel absorbs water and deteriorates some fuel lines and gaskets.
I can't see any of the products helping with #1 since they cannot stop evaporation and there aren't enough added distillates in that little bottle, although it's the primary ingredient. Seems adding fresh gas (maybe premium) to the old gas is the only way to "freshen" it up. (2) I'll have to take their word for it as I don't understand fuel antioxidants. Looks like Stabil may have an antioxidant in it's secret sauce. Star Tron uses an antioxidant enzyme and SeaFoam has none that I can see. (3) Ethanol itself is a great solvent which is why it eats some plastic/rubber fuel lines and gaskets and pumps. It would seem that Ethanol cleans your carbs/injectors while running, but I'm guessing it's not so kind while sitting in the fuel system for 6 months since it can absorb water.
So if you run your boat frequently and keep your gas fresh, how much of this stuff do you really need? This is just my $.02 worth, I don't claim to have a full understanding of fuel treatment.
Here is the breakdown of what's in Stabil, Seafoam and Star Tron.
Stabil
Petroleum Distillate - 95%
Additive Mixture - 5%
Seafoam
Pale Oil - 40-60%
Naptha - 25-35%
IPA - Isopropyl Alchohol 10-20%
Star Tron
Proprietary 0.5%
Solvent Naphtha 95%
MSDS sheets here
Stabil SeafoamStarTron