2nd, 3rd, 4th, and 5th on the Swallows and Amazons series (that's one vote from each member of our family). My wife has been reading Ransome's books to my 5 1/2-year-old son, and he's eating it up. Even our 2-year-old daughter seems to enjoy it ("What John doing, Mommy?"). Ransome really captures a child's vivid imagination and the seriousness of play.
Other possibilities:
--Jessie's Island, by Sheryl McFarlane and Sheena Lott. Not sailing exactly, but a really well-done exposition of the joys of exploring a Canadian Puget Sound island. And beautifully illustrated. Great for younger kids
--Danger, Dolphins, and Ginger Beer, by John Vigor. We just found a used copy, and haven't read it yet. It looks to be in the same genre as Swallows and Amazons, and we've appreciated Vigor's other writing, so we're looking forward to this one too.
--The Annapolis Book of Seamanship. Don't ask me why, but it's been one of his favorite books for months now, and a frequent request for bedtime 'stories'.
Aside from books, the things that have probably most impacted our son's sailing addiction are getting to helm our little skiff, and watching America's Cup on YouTube. Say what you like about AC34 (and there's plenty to say), but he's pretty well justified in finding watching 72-foot foiling cats a pretty exciting pastime.