It is a standard comes with the boat 6 inch round cover. O ring seal, plastic inserts in holes that should provide sealing. I'll look into an Armstong plate. Thanks.
Thanks for the insight. I have resealed the access plate and thought that would have done it. I bet it could be the top of the bracket, although there is a swim step above it. Definitely rainwater, stays full with no visible leaks out. Thanks.