I always use the middle octane (89), on the advice of my boat mechanic. I run both a 115hp two stroke and a 9.9hp four stroke off the same tank (through a water seperator and check valves).

I think the important thing is that you use a fuel stabilizer, something like OMC 4+2. I'm told fuel can lose its octane and becomes "stale" within a matter of a month or two if you don't use the stabilizer... something that could easily happen in your boat's tank over course of the winter.

I also always fill up (got a 35gal built-in tank) after every trip, this is something I learned from flying airplanes. If you leave airspace in your tanks, you're just inviting condensation buildup, which results in water in your fuel... so keep your tanks full of fuel, and you get much less condensation. On airplanes, there is a fuel sump drain that you drain off and test before every flight, to see if there's water in your fuel. I haven't yet seen a boat with this feature. :-)

Oh, and another important thing, don't buy cheap gas. Cheap gas (ie. Safeway, Costco, Arco, etc.) has a high alcohol content. You don't want alcohol in your fuel. Alcohol attracts water, and it also eats away at the rubber components in your fuel system, like all the o-rings and hoses. Spend the few extra pennies per gallon and buy quality, non-alcohol fuel, from dealers like Chevron or Texaco.

-N.