Uber is best bet, as public transportation sucks.
I would be dropped off at B Ocean resort or the next door Marriott, so you have a place to dump a bag at bell desk. And you can use the pool shower to rinse feet from walking on the beach.
I would think you are good heading back at noon. You will have already shown passport on check in. Security lines usually aren't bad.
One thing to consider is the 17th st causeway bridge. While it is very high (55ft.) and does not open all that often it can open on the hour and half hour. If you hit a bridge opening, that could add 20 minutes to your commute time. This time of year, you will have large sailboats headed north.
But as long as you are through there before the hour or half hour, you are okay.