Sea Chest
The sea chest on my DF44 has a 1" thick piece of Lexan material as the cover, and is through bolted through the bottom flange with 3/8 s/s bolts & nuts, 3" on center. I don't think it is readily going to " blow off" and sink the boat.
Most of the Defevers have the sea chest top just above the water line. If we are fully loaded with fuel, water and supplies, the water level is right at the top.
To the OP, if you are considering buying this vessel, I would factor in upgrading and replacing all the thru hulls and sea cocks on that sea chest. From your pic, it seems as though there are some elbows attached to the thru hulls and then the sea cocks ; a point of failure.
Bill