Bob,
This looks like no biggee. If it were my boat, I'd live with the ugly for awhile, chalk it up to a fishing boat. Then when you're tired of looking at it, pull it (hatch cover) flip it over, dig out the rot, and put a heat lamp on it. Then fill the void with epoxy and filler (dried cypress slivers) from the backside.
......
Cheap fix. Spray the backside of the hatch flat black............go Ninja!
Keep us posted on the fix.