While it depends on the size and weight of your pontoon, it’s always good for stealth if you can go electric, with a thrust suitable for the size of your pontoon and your desired speed. However, if you have long distances to travel, there really is no substitute for gasoline. The absolute best way to determine the best motor size is to check the manufacturer's recommendations and stick to them!  Being underpowered can be a pain. Being overpowered can be downright dangerous.