It takes A LOT of rain water to sink a boat. I'm not aware of boats getting "glued" to the bottom at low tide in a way that would hold them down in opposition to the buoyancy forces when the tide comes in. Maybe a fin keel boat could tip sideways, and a storm with big seas could wash enough seawater into the boat to sink it.
Or, some big motorized-holes-in-the-water came through, throwing their tsunami-sized wakes at her while she was vulnerable. At the wrong time, it wouldn't take much to flood the cockpit & overwhelm her. Can you tell how big the drains are, and are they completely clear?
When I stood on her deck leaning against the shrouds with her moving under me on the water, this evening after she was free of the mud, it felt liker home. sailboats have that effect, even when they are in need of help.
Ain't that the truth! Careful, Jack,- she's seducing you. Did you get the info about the designer/builder? I can see added attraction in saving a rare breed of boat, if she was basically sound in design & build. Kind of a knight -you- saving a princess -the boat- from a dragon -the sea- story. Romantic? -Yes. Possible? -Probably. Practical? -Maybe not so much.
Too bad you don't know too many people there, you might be able to pull a Tom-Sawyer-and-the-picket-fence. Maybe some of the folk at the marina would be willing & able to help.
It just occurred to me what this reminds me of: Mal Reynolds finding 'Serenity', on that fun old science-fiction show "Firefly".
If you could go into this with that mindset, and sustain it, you might pull this off in spite of the steep climb it looks to be.