That is why you need to carry a big underwater knife, to kill the clam and pry your foot loose.
Not that it ever happened, but I do miss my original 1970's 12inch, US Divers leg knife.
1 1/4 to @1 1/2 inch hammer head hilt, serrated back, strong enough to pry a safe open, and damned cool looking. You needed less lead carrying it too....