It will not rain all day (unless there is hurricane or tropical storm nearby).
Florida in the summer is very tropical, which means you will have very humid partly cloudy to sunny skies in the morning and then a thunderstorm or two will roll in during the afternoon. They usually do not last long. After the storms roll out, it will again be sunny or partly cloudy and very humid. It is really no big deal, and sometimes the rain is even welcome to cool things off.
Do NOT look at the weather reports. For all of my Florida trips and for my numerous cruises to the Caribbean, the weather always called for thunderstorms every day. As stated above, this does NOT mean all day rain. I always had great weather, albeit with some thunderstorms here and there. Mostly sunny.
Don't worry, it will be great!