Finally did the man-overboard test!
I could have spared myself the trouble of trying to make the deck strong enough to carry me. Once overboard there are no way to 'restore to default'. Sure, I could fill for and aft with polystyrene-foam, balloons or something equivalent....
If the overall structure was rigid, I could turn it up side down and empty it for water .. and then climb on-board from say the rear. But it's not rigid.
Well, it wont spoil the fun for me on the water .. on most of my local water ( 57° 2'41.63"N, 9°51'3.89"Ø) I can walk home from a miss.

... ready to build a new ..