Rear Differential

Recently got a 1996 Honda Fourtrax 300. It’s in great condition and is snorkeled and the only problem I have is getting water in the rear differential. The seals and all are good. I have had several people tell me that all 300’s leaked. I also had a guy tell me to pack the rear end with corn head grease. I ride a pretty good bit cruising around the farm and every few weekends we will do some serious riding in water and mud. Does anybody know anything about using grease? I really don’t want to replace gear oil every time I take it through water. What are the pros and cons of this?