At some point, when we are/were learning to waterstart, all of us have had the "buried clew" problem.
It still happens to me when I have to waterstart in deep water or big swells.
As Ken suggests, using your arm (with your hand in the rear foot strap) can rally help, even with fairly large sails.
But overall, swimming out to above the boom, to lift the rig enough to get the wind under it, working on your sail control (to keep the wind "lifting" rather than stuffing the clew), and keeping your alignments (the board, the rig, and the direction you are loifting/controlling the rig) nearly always works, when nothing else will.
You have to do it alot, and the more you do it, the better you get at it. Vidieos, and instruction can help
some, but experience (lot's of it) is what ultimately leads to success.
Hope this helps,