I mainly shoot in tropical waters, so its a bit different, since there is less hesitation to go into the waters.
That said, the FL really depends on the situation, whether the beach has foreground interest and is there a bkgnd object
I find anything from 12mm to 31mm to be fine, just need to look for the foreground interest
The 12-14mm type lens looks down and to the side due to the UWA and this can serve to catch foreground interest which may be harder with a tighter fov.
12mm
The perspective exxageration can make a simple item (like the formations on the sand in this photo become part of the image.
31mm or 28mm gives a more wide/normal perspective in that it does not push distant objects away as much
This with the FA31ltd
24mm is a great all rounder.
Tripod will be a must to blend in a wave or some form of repeated wave action