Before buying a live-aboard boat, says Open University instructor Tim Ehlen, you must consider:

Type of boat:

* Sail: "You need a longer boat to get enough living space."

* Power: "Good room for living, but they use a lot of fuel and can be more expensive."

* Houseboat: "The most houselike--the poor man's yacht. Meant for protected-water cruising such as lakes and rivers. On the Chesapeake, they could be uncomfortable--and dangerous--from pitching and rolling in a wind."

Type of construction:

* Fiberglass: "Easy to care for."

* Steel: "Solid, but it rusts."

* Wood: "The most natural fiber, but it requires a lot of maintenance."

Type of engine:

* Gasoline: "Requires a lot of maintenance," but most reasonably adept people should manage. Be alert to the danger of a fuel explosion if proper care is not practiced.

* Diesel: "Costs about twice as much but requires practically no maintenance. When it does, you need a mechanic. If I had my druthers, I'd have a diesel." Diesel fuel is less volatile than gasoline.

When buying a used houseboat, Ehlen advises:

* Look for one at least 40 feet long for sufficient living space.

* Contact a broker, or buy it on your own after you have taken time to compare different designs.

* Expect to pay several hundred dollars for a survey or inspection of the boat's non-mechanical parts. You probably will want to have it hauled out of the water.

* Take a mechanic along for a sea trial. "Find out how--and how well--the boat works."

* Sign up for a safe boatsmanship course from the Coast Guard Auxiliary, U.S. Power Squadron, Red Cross or state or local agency.

* Spend time aboard first to see if the life style suits you before committing your money.