Great response Ray, thanks. Below that are additions by two professionals who work in the field and literally live or die by this technology. Gotta love SailNet.
I would only add the emphasis again that SCUBA certification is necessary AND worthwhile. It's cheap (usually under $150) and they teach you the stuff you didn't even know you didn't know that WILL kill you if you didn't learn it.
Additionally, getting a small pony tank + reg could be one option for additional safety instead of the stored air in the compressor tank. More redundant, but much more expensive and filling the tank is an issue as you need a SCUBA compressor. There is also a self contained unit called Spare Air that you could take down with you also, but again at increased cost and difficulty filling.
I do think the DIY can be done safely if you really do your homework
, but as has been mentioned before, a purpose-built hooka system off E-bay is probably the safest/cheapest/best option proposed so far IMHO.