I have a pair of exhaust tips I want to install on my boat. I was just wondering do I just screw em right through the boat, or do I use any sort of gasket? I was told just take 3 stainless steel screws and thats IT!

Typically they have a mounting ring on the inside that accepts the bolts, Most cases you would silicone them in along with bolting. It depends on the manufacture some just screw into as well.

I just put new tips on my boat and I through bolted them with 3" long machine screws and fender washers and nylon locknuts on the inside. I sealed them with Life Caulk. You can get it at most marine stores. It's the kind that's made to seal metal to fiberglass. I don't have any leaks and the new tips look great.