Matt the norm is usually aluminium 80 cubic ft air tanks, stored upright to prevent any rust/debris from entering the tank neck valve. Most boats I seen are stored on the gunwales in a storage
rack. There are racks made for boats that are used commerically in the dive industry. I suggest buying
that system if possible. I have only came across steel
80's in the Florida Keys
at one diveshop. May I also suggest that you buy an O2 provider unit from DAN and receive instruction on how to use it properly, its very important to have O2 aboard to start first aid to a injured diver...
I have dived many sites around the KEYS and some around Bimini Islands
... BEWARE that most third world countries do not have recompression chambers so DAN diver's insurance
is a must to anyone who dives... They cover the complete cost of transportion from the sailboat to a recompression chamber by the quickest way possible. It's not usual to have a $50,000 dollar bill transporting a diver to a chamber... It's not worth losing a sailboat over when DAN's insurance
is so low price
Just for anyone's information I'm certified Extended Range Deep Technical Diver by PADI.