RTV silicone has no business below the water line of a boat! Sheesh.
5200 is what I'd choose... not sure why you'd want to do this job again in the future? 5200 would be a PITA to remove if you did, but thats because its good at what it does (sealing below the waterline). If absolutely necessary, it could be undone though.
3M 4200 or Boat Life Caulk are 2 other slightly less permanent options.