Look at a chart of the area, examine where you want to go, and decide if your journey will expose your vessel to the prevailing northerly wind. Ask how many engines power your chosen boat. Two is safer than one. Ask what is her range and speed. Ask how long the vessel is and how wide her beam.
Long vessels cope better in rough seas.
Beamy vessels have more space but tend to roll more. It is almost impossible to tell how dry a ship is until you get to sea. Bigger vessels are not necessarily better. They are usually more spacious, but naturally cater for a larger number of divers. Some, too, have magnificent saloon areas but very cramped aft decks where you have to struggle to kit up.