I'm with Peter HK on the ranking: You can skip book two, Swallowdale. Very little sailing, lots of boring exposition.
Jump right to We Didn't Mean to Go to Sea and then Peter Duck. These are great books, I've enjoyed reading them to my kids and thanks to SteveW one of my current boats is named Wildcat after a boat in the series.
Edit to add: Steve, I did change the name to "Kitty", like they do in the movie, just to prevent the giggling from the boys every night when I was trying to read them to sleep!
My favorite funny moment from the book is when the youngest makes an illustration in the ship's log, a page entirely full of black ink, to show what it looked like at sea at night.