There are "Humps" off Islamorada and Marathon Keys. Distance depends on what channel you run out of.
From the Channel 5 bridge, it's about 22miles to the Islamorada Humps if I recall, closer from Widley Key.
From now to early June the weather is pretty predictable -- fronts coming through and windy. You will get a break in the wind between fronts -- you just need to learn the pattern.
Once the jet stream heads north in early June, the winds die down, but then it's T-Storm time. They can pop up any time and turn ugly fast. Thats why "smart" anglers head out at first light and call it a day before it's too late.