You may already know about this, but Tom Yost has info about adding a hatch opening that closes like a dry-bag. It's glued vinyl construction suits Tom's PVC skin material, but I don't see why a similar thing cannot be sewn together from a waterproof material & sewn to a fabric deck.

I have also seen somewhere on the www where someone made a hatch like a miniature coaming, using wooden embroidery hoops sewn to the skin, and a fabric/neoprene hatch cover sealed by a bungy cord or drawstring.

Maybe a combination of the 2 would work best...

Sew a hatch ring to the deck fabric, then sew a dry-bag type of roll-up closure to it ?

You could also add your original idea of a sea sock for hatches if you want additional sealing.

: One of my students in the Yost SOF class had a wonderful idea. He wants
: watertight hatches on his SOF that he'll build after this class and we've
: been discussing options. He had the idea of using something like a sea
: sock for the hatches, only the opening would fit under a screw in Beckson
: hatch that could be fairly easily added to the fore and aft decks. With a
: few tabs sewn into the bags they could assume the rough shape of the
: compartments they are in. Sounded like a great idea to me

: Watertight hatches without the bulkheads that are a pain to add to a SOF.

: Bill H.