IMHO, your least expensive option is to buy a house with a dock in the backyard. You may need to look just a little bit south in Palm City, and Stuart. Look for houses with "deep water access" or "no fixed bridges". Houses that are on a canal are less expensive than houses on "wide water". There are a lot of foreclosures in this area that are driving down home prices. My nephew looked at a foreclosed house with deep water access, 3 br, 2 bath, 2 car garage and a dock that had an asking price of $360K. It went to auction, while it has not yet been recorded, rumor is that it sold for $375k.
Slips can vary alot depending on whether they are behind someones house or a marina, and how far they are from the inlet. I believe the high end would be $12/ft/mnth close to the St. Lucie inlet.
Insurance is crazy. $5k - $7k per year depending on your history.