As stated above it is a fuel system cleaner but it also decarbs an engine, which is a big benefit for an outboard, especially a two stroke. There are several u tube videos that show the interior of an engine before and after using seafoam(with a camera thru the spark plug hole), it works. I don't know that it is any better than the Mercury product but I suspect for $6.50 at Walmart, it is cheaper(just a guess). I free pour when I mix in a gas can but I would suggest mixing by the directions if you are in doubt.