I 've been running 93 on the street for my 10.5 comp motors. At a recent race I bought 90 oct at the site with no ethanol. I was a little nervous but finished without blowing up. In the future am I alright to run the 90 ethanol free? Is the 93 ethanol the same as 90 reg as I was told at the races by the seller?

Ethanol is hydroscopic, leans motors out ever so slightly and doesn't belong in a marine engine in my opinion BUT OCTANE is OCTANE, your motor either only needs 90 octane or it doesn't, IF it is ok on 90 octane and you can buy that 90 octane with no Ethanol than it would be the fuel of choice, however if your motor actually needed 92 or better than just because it contains no ethanol doesn't mean 90 octane is a great substitute for it just because its ethanol free. My own motor needs 92 or better to run in boost, I would have to take the ethanol 93 over the non 90.
Do your motors have iron or aluminum heads? With 10.5 and aluminum heads you could maybe run 90 octane safely IF your total timing isn't right at the edge ,also depends on cam specs/cranking cylinder pressure, efi or carbed, knock sensor?

They are iron heads, and i put 72 gallons in a 140 gal tank on top of 93 street gas. Am I just lucky. I haven't run the boat at all since coming home and have not checked the plugs.

I talked to a guy at a local marina. They have what is called 90+. He said it is ethanol free and when tested is 93. I am no expert, this is just what I was told.