Depends on how deep the person is, but since they're scuba diving in somewhat shallow waters (no wet suit, at the bottom but can see, etc.) not a terribly high amount.

If we assume 50 metre depth and 1 metre diametre plug:

p=ρ×g×h+p0 = 1022 kg/m3(9.8m/s2)(50m)+101kPa = 602 kPa or 87 psi for you Americans.

Area plug = πr2 = π(0.5m)2 = 0.785 m2

F = PA = 602 kPa x 0.785m2 = 472.6 kN or 106 238 lbf

So, far above what a person could pull, but not absurd amounts considering the sizes involved.

Edit: fucked up my units and force formula, whoopsie.

Crab pipe. 'Nuff said

Don't forget the sequel.