...my understanding is Sherwoods have a 1st stage dry bleed system, which you can see in action by watching the tiny stream of bubbles trickling out of the 1st stage while it's is in use under pressure, so if the regulator is underwater without positive air pressure, water will back flush into the 1st stage. ( I've never owned a Sherwood reg, but know divers who do.)
The Atomics regs don't have 'issues' with the 1st stage, they do have 'issues' with their 2nd stages...they have a seat-saver 2nd stage that relaxes under zero internal air pressure, allowing water to back flush into the regulator.......Poseidon Jets will do a similiar thing.....the balloon valve (condom) inside deflates under zero internal pressure allowing water to back flush into the 1st stage.
Karl