We are all booked on a refresher in the next few weeks. What T.Annimal said is quite right but I would just like to add another unseen danger that cost 2 fish farmers their lives in our area about a year ago. Whilst inspecting the bilge one boy collapsed and his two pals went down to get him and they promptly collapsed as well, long story short the rust in the tanks had used up all the oxygen. I don't want to say too much as a lot of gossip is said regarding incidents of this nature but make sure any tank is well vented and you carry an oxygen meter.