The best way is to find toluene. It has been used as an octane booster for many years. It was used in fighter planes during WW2. There are charts all over the internet on expected octane vs mix ratios. The only problem with it is that you have to find a supplier who is willing to sell it to you in bulk. It is way too expensive by the gallon can. The mix ratios go to about 30% if I remember correctly but it really does increase octane by a significant amount. It is as close as you can get to racing fuel. Some of the octane boosters available actually work, but most are crap. When it comes to a high dollar, super heavily loaded boat engine, I would be very careful about what fuel I'm running if there was even a slight concern about detonation. 100LL would be my second choice, but I have heard of some lead fouling problems in automotive applications. I would think that it should fare well in a boat. In any case, I don't know what you're doing with your boat, but if it's strictly recreational, turn down the boost and run gas from the station (except for the occasional shot of race gas and thrill of course).