I used to be a Life Guard. I'm also a native Floridian. Daytona would be the best place if you're single. But because you're not and have a child, I'd recommend Sarasota because they have the better schools out of the 3 you mentioned.
The problem you're going to find is that all beaches are the County Government responsibility for Life Guards. In Daytona, they are also Police Officers, so they generally recruit previous officers, rather than WSI credentials. Many Life Guard jobs around the state are seasonal or part-time only, so they are second jobs for many people and don't have benefits for you/your family.
Because County governments are funded in Florida through property taxes (we don't have state income taxes), they are now facing severe budget shortfalls due to the real estate market crash and are now scrambling to cut services (and employees).
You definately wouldn't want to relocate your family without landing the job first, and your best bet would be to check for job announcements posted on each of the county web sites.
I also don't want to be a downer, but remember that most teens who live on the Florida coastline are usually active on local swim/water polo teams (as hockey is in Minnesota), so you'll be competing against many seasoned local applicants.
