Saw an airboat on tv the other day with what looked like a container of windshield washer fluid or antifreeze. I’ve googled images of airboats and only found one other picture (in the first few dozen) with a similar container. It’s mounted next to the engine, but outside the fan housing. Here’s a picture Photobucket | Make your memories fun!
Looks like an antifreeze reservoir… but that’s just a SWAG.
Cooling system overflow tank. A lot of boats use aluminum tanks instead of plastic.