Thanks Alexandre, Mark and Andrew for your replies. Each was very helpful. As to why I am concerned? This is such a critical system and a leak could have such dire consequences, even with the sea-cock closed slight leakage past the valve will eventually overflow the top of the sea-chest (as most have noted). I feel very uncomfortable not having the lid in place.
I have located a diver and I am trying to find a picture of the lid to email to him so he knows what he's looking for/feeling for in the murky water, yet having scanned every photo on this site and all, 2,400 pictures I have taken of my boat and its systems, I can't find a single picture of the sea-chest lid or even of the seachest/thru-hull. Boy, it just doesn't get any photo respect.
Could someone take a pic of the sea-chest lid and one of the sea-chest so that the diver will know what he is looking for and then my caretaker will know where to put it. Please post here or just email the photos to me at garysilver at mac dot com.
Thanks again for the replies, the information, and photos.