Osmosis shouldn't be a problem - remember the hull was designed to be wet in there.
What might be more of a problem is the scratch marks on your engine leg from the crabs & other assorted creatures that decided to crawl in when the tide was out & get flushed out as you open the throttle!
Transom drains / trunks are as variable as the people who fit them. From memory the floor will be below water level when afloat on a mooring (esp if you got a 60 on the back!!) so you'll either need an auto bilge pump or one of those nice reliable non return valves
so it empties when the tide falls. Personally I would replace it with a trunk, then worst case you open it when you leave the mooring (and remember to wear wellies or a drysuit!)