I have a 2004 Gulfstream with a single engine attached to the Gill Bracket. The Gill bracket has a hatch on top and a drain valve on the bottom. Water gets in but does not drain, therefore the leak must come from the circle hatch. What would be the best sealant to use around the round metal hatch and in the screw holes?
There are other places where water can get in. It could be that 'cover' but it could be a pin hole in the metal or a leak where the bracket bolts through the transom. If you ca get the boat out of the water, one test would be to remove the cover and fill the bracket with water and fill it all the way. Put the cover back on and look for seepage. If you don't see any, let the boat sit for a day or more and then see if the water level changed. If it did you have a leak somewhere. You should be able to find it by wiping all edges and seams with paper towels to see if they get wet.
If in the water you might want to try using a piece of clear plastic sheet to make a 'cover' for the access plate. The plastic should be wider than the access cover. Use some silicon caulk to glue the plastic to the bracket in effect sealing it off. Run the boat as usual and check for water getting into the bracket. If the bracket is filling up, the access cover isn't your problem.
If you want to try sealing the access cover, use something like Life Caulk. Do not use 4200 or 5200 type of adhesives.