ski8um

New owner here, so bare with me.  I have a 2003 Utopia 205 that I am replacing the bilge pump on.  The new pump is a newer style than the old pump and I have to remove the old one completely.  For some reason, the old pump base was secured to the bottom of the boat using rivets.  Is that normal?  I doubt it, for the simple reason you cannot just remove and replace it.  Either way, once I pull the plastic peice out, can I just install the new one with screws?  How thick is the fiberglass in that area?  I don't want to creat a hole that water can leak through.  I'm assuming it's thick enough, but am a little unsure since they used rivets on the old one.  Any help would be greatly appreciated.