Blue Star Ferry
This
is pretty good but it only stops at Leros three times a week (ditto back)
and leaves late, 10.30 or 11.30pm depending on the day.
Kos is lovely, with a castle, Hippocrates Tree, loads of shopping and sightseeing. There are some great beaches - we tried Beach Heaven. Rows and rows of (free) beach beds - but with nice young Swedish men fetching and carrying food and drink(!) He called himself a beach boy.
The harbour is right next to Kos Town, so well within walking distance. Taxi from the airport (around 23€) 20 minutes to Kos Town.
The really good news about the Blue Star is it's so big it doesn't have the problems with wind that the lighter craft do.The Blue Star still takes an hour and a half but you have a nice day in Kos. We've done it because there were no catamarans or hydrofoils in 2004. Although you only get to Leros at 1am it's a nice trip, good bars and if you don't mind kipping on a chair in the bar...
This is a link for all ferries, hydrofoils, catamarans http://www.gtp.gr/