The attributes are shown below–not exactly in priority order, although those listed first may be deemed more important than those listed later–because for the most part they are integral to the boat and couldn’t be added by later if they wanted.

• Affordable. Can you afford to buy and maintain this boat without borrowing or overextending ourselves financially?
• Good survey. Sound condition and structure, and a dry boat. No need to keep everything wrapped in plastic.
• Good ventilation without air conditioning
• Heavy displacement cruiser, best with with a full keel and attached rudder. Able to take care of you in bad weather while you hunker down below.
• Inboard diesel engine powered at not less than 3 hp/ton. Sufficient power to make your way motoring or motor-sailing when necessary, or to power up and get out of a tight spot.
• Plenty of accessible and well-ventilated storage. This is your home, so you need enough room for books ( and other comforts, plus all the spares, tools, etc. for blue-water cruising.
• 6″ Bulwarks with scuppers. Good solid footing while walking around the deck, and good drainage in downpours or shipping green water.
• 8 strong through-bolted deck cleats. Strong attachments for docklines and anchor snubber.
• Squared off coach roof/cabin top. Strongest configuration for a knockdown.
• Dual bow anchors, one with minimum of 200′ chain. Second anchor for storm conditions, and plenty of chain for normal conditions.
• 100 gallon fuel tank. Enough to give a range of at least 500 nmiles under power.
• 2 50-gallon water tanks. Enough to last 3-4 weeks without rain catching, or backup in case one tank gets contaminated or emptied.
• Small aft cockpit with 4 drains and strong padeyes for attachment. Comfortable and safe for whoever’s on watch, and safe in a seaway, with ability to drain fast if much water is shipped.
• Aluminum keel-stepped mast. Minimal maintenance and more support than deck-stepped.
• Good handholds and footspace on deck for moving around. Essential for safety.
• Good handholds and headroom below. Headroom for a 6′ person, and solid handholds for moving around below when the seas are up.
• Manual windlass. A bit more work for anchoring, but no likelihood of sticking in a dangerous situation.
• Dodger, splash cloths, and bimini. Dodger with easy visibility forward to keep the wind out of the cockpit, and along with splash cloths keep crew in the cockpit dry when water is shipped, and Bimini to shade you from the tropical sun.
• V-berth with double bed on one side, all berths accomodating 6′. Good space to snuggle, and comfort for tall crew.
• Manual bilge pump in cockpit and below as backup to the automatic bilge pump. Ability to manually empty the bilges from the cockpit.
• Starting battery separate from house batteries with a battery monitoring system.
• Enough electrical storage to light and cool the boat, plus run basic electrical equipment without excessive recharging requirements.
• Easy way to tell the condition of the batteries (input, output, voltage, status)
• Autopilot. To relieve the helmsman when under power.
• Windvane. To relieve the helmsman while sailing without draining the battery.
• Swim ladder An easy to drop and retrieve swim ladder on the side of the boat.
• 3-burner propane stove with oven. Able to cook pretty much whatever you want.
• Cabin heater. Keep you warm in cold nights or in the winter.
• Liferaft, MOB module, flares, fire blanket, propane and CO detectors, and fire extinguishers. Essential safety equipment.
• Manual and sea water pump in galley. Ability to pump fresh water if the electric pump fails, or seawater to minimize fresh water use.
• Radios–marine SSB with ham bands and GMDSS VHF. Essential communication equipment.
• Dinghy with motor. Ability to get around when at anchor.
• Radar. Essential for navigation at night when near land, or in shipping areas, or of course in fog. Also a tremendous assist when approaching an unfamiliar harbour with a hard-to-find entrance, or entering or leaving an anchorage at night.
• Wind instruments (vane and speed) and depth sounder.
• Bug screens on portholes, hatches and dorades. Let the air in, and keep the buggers out.
• Perhaps the most critical component is the crew. If the crew is sound, they will know how to sail offshore and also if the boat is suited to the trip.

Source:

Book by John Vigor “The Seaworthy Offshore Sailboat: A Guide to Essential Features, Handling and Gear.” At the front of this book, there is a really neat 100-question test/quiz which gives you a quick determination of the blue-water seaworthiness of you and your boat.