I guess you are right one could do a refresher in the classroom after 2 years and then go back in the water the following year or some timing like that. I used to use my survival suit which I fly in and stayed 99% dry which cut down on the drag of undressing and all that.
Stass is where you take with you a little Oxygen bottle like James Bond or Pamela Anderson on Baywatch duty which is like a miniature Diving outfit. So you can breath for 5 minutes under water. i have done my HUET once with it 6 years ago because it looked like we were going to get it but nothing came of it though I am told Aberdeen are thinking about it again so we should get it in the next 100 years then.
The two Dauphin's in Plymouth who who supply the Navy ship's with personnel and stuff have STASS because the Navy requires them to have it. So it is perfectly possible to have.