I personally hate underwater thru-hulls as well (removed 7, stuck with 3, but heavy duty Groco, perfectly installed
One way to do it would be to have a sump, which you would have pump out regularly (perhaps with automatic electric
pump). The downside of this is the stinky sump. You would have to clean the also fairly regularly, since food
and all the gunk that does down the sink drain would leave offspring in your sump tank. I thought about it but on our boat, we also use the sink drain as one of the cockpit
drains, so it seemed more advantageous to keep it.