There are a lot of different motor oils available for boats. Before you even start looking, you should make sure you know what to be looking for, based on the type of engine that your boat has. For instance, a 2 stroke engine requires special 2 stroke oil, so if that’s what you have, you should make sure that that’s what you’re going after.
Once you’ve figured out exactly what kind of oil you need, you can get into the specifics of brand choices and things like that. There are many different brands of motor oil that you can choose from. You could take a look at Yamaha or Mercury oil, but personally, I prefer Optimax oil.