I'm no rocket scientist, and I know almost nothing of SF Ops.
But at a rough guess, I'd hazard a guess at "being in the Special Boat Service is dangerous" and leave it at that. Other than getting wasted in the Jolly Sailor on Poole Quay, everything they do is flipping dangerous.
Something to do with old rebreathers only working above a set depth and water pressure on the body
Also your dry bag turns into shrink wrap and you can't kick for the surface due to pressure and so just sink.