I've 'cheated' and used both of the following short-cut methods (versus setting up a separate tank):
1) Pull your fuel / water separator and fill it about 75% with SeaFoam. The SeaFoam will mix with the gas getting pulled into the f/w separator and also with the gas in the VST tank if you have a Yamaha outboard.
2) Using the DeepCreep spray, just remove the black air intake cover on the front of your Yamaha and spray the Deep Creep into each cylinder like you are fogging the engine. This is definitely the easiest method unless you are changing F/W separators anyway.
-- Tom