It is true that you will pay $3500 on up for a scuba compressor that pumps safe air. But a proper low pressure compressor system using the right hose, regulator or mask and filtration will cost as much as $1000 ... even in the PI!
Lots of things have been brought up here and some of them are gold and some pure BS.
So, lets start with air source.
- A scuba tank with a long hose is great since it is filled with filtered grade E air. The drawback is that most second stage regulators (the part that goes in your mouth) and breathe harder the deeper you go. Additionally, you have no indication of how much air is left in the tank until you run low and it gets REALLY hard to draw a breath.
- Next is a compressor. You can use just about any compressor that can put out the proper flow rate. Remember, when you are underwater pulling a wrench or scrubbing the bottom, you are breathing harder than if you are just sitting on the deck. Plus, even though it is pretty shallow at the keel, the increased density of the air means higher flow rate. Commercial compressors for 20' or less are 12 CFM or bigger so go with something that size.
Next is filtration.
- The act of compressing air increased the density. But, since water is not compressible, the humidity level is greatly increased. Plus, there is always a little carryover from whatever lubrication is used in the compressor. so, a proper moisture separator will remove a bunch of the moisture and then you follow with a chemical filter bed. Additionally, any compressor will impart an odor/taste into the air so you will need a charcoal filter as well to sweeten the air. And ... NO ... those cheap plastic air filters just don't cut it.
- It is important to have a backup air source should your compressor shut down. Most LP compressors are available with a holding tank. If you are pumping away and working at depth and run out of gas, propane, electricity or squirrel food, the compressor stops. Believe or not, you can generally hear a compressor running through the air hose and, if the sound stops, the volume tank gives you enough to safely make your ascent.
- Standard SCUBA regulators are built to operate at from between 120 and 175 PSI and are pretty finicky. If the feed pressure drops a a couple of pounds, it becomes a little harder to breath. For a low pressure compressor, there are special compressors meant to operate at lower pressures and are a little more difficult to find.
- The feed hose to the regulator needs to be made of the proper material. Cheap rubber hose breaks down rapidly and tastes terrible! Cheaper vinyl hose tastes terrible all the time! Get proper breathing air hose. It ain't cheap but makes working under the keep a LOT more comfortable.
OK ... all that said ... forget the DIY hooka system. You bought a boat to travel and enjoy yourself. Learn scuba, bring your gear aboard and dive in cool locations.