I don't think that the exhaust on a model boat would have enough thrust to make any difference. These aren't Harriers or Top Fuel Dragsters. T/F dragsters and Funny Cars do use the exhuast pressure for free downforce, up to 600 lbs of it, this is the reason why the zoomies are angled up and back.
If I can stop the engine just by placing my finger over the exhaust, that kind of shows me that there isn't a lot to work with there.
It really can't hurt to try it though.