Brushcutters

I have the 336fr model.  works great. I will be clearing some invasive species shore grass in the very near future. .The grass (which grows up to 7 feet high) goes out into the water several feet from shore in about 3 to 4 inches of water max. The grass growing on shore obviously wont be a problem, but i would like to know if im safe using this unit cutting the stuff at water level. Even if im very careful, im sure the cutting head/gear assy will at some point be in the water..Is that ok as long as i check the grease reservoir after im done to make sure no water has gotten inside it? thankyou for your time al in kalamazoo mich.