Seafoam is NOT fuel stabilizer. Sta-Bil would be the #1 brand of fuel stabilizer, and it works very nicely. While Sta-Bil is largely naphtha, basically gasoline stock, the "secret" ingredients in it actually stabilize the gasoline compounds. (Gasoline typically is blended from 60 different components.)
Seafoam is a mixture of "IPA" isopropyl alcohol, light mineral oil, and naphtha. In other words, alcohol to clean things and "explode" to steam clean the cylinders, gasoline stock so it keeps running the engine, and extra oil to lubricate the parts which the alcohol has just steam-cleaned and left bare of lubricant.
You can make up your own mixture to do the same thing, read their MSDS and you'll see the proportions. Personally? I can't see any reason to use it unless you are trying to use that steam clenaing to blow out carbon on engine valves, or something like that. And, even a pot of old coffee will do that.
Ethanol in gasoline is, and for many years has been, not only normal and common but required in much of the US. I'd be really surprised if a Tohatsu engine could not run on standard E10 (10% ethanol) gasoline, and consider it unsuitable for use or sale in the US market. Was this a gray-market engine not sold by an authorized US dealer?
Gasoline without 10% ethanol is impossible to find in many states. You can buy "100 LL" aviation gasoline at many airports--but that's also LEADED fuel, and you can expect to pay $4.50/gallon for it. Some marinas might carry "real" gasoline...but unless you live near one, that's something you can't expect on any regular basis. (And I went looking for it, because the "winter" E10 blend really kills mileage in my car, but in some states "real" gasoline simply won't be found at any conventional gasoline station.)
Personally...I'd break it down again, make Real Damn Sure everything was surgically clean and correct on the reassembly. If rubber parts were left in contact with fuel over the winter, or fuel that was not stabilized was left in the engine more than 90 days (pump gas breaks down after 90 days), it might just need a good cleaning, some rubber replaced, or something may have been assembled just a bit off.
Using E10 shouldn't be the problem though. Not if you follow the "rules" about using it up or stabilizing it, and not if the engine was sold for the US market. You should be able to contact Tohatsu directly to ask them about using E10 in the engine. Tohatsu Outboards: Authorized North American Distributor for Tohatsu Outboard Motors, Parts, & Accessories.
should get an answer for you, for sure.