I have lived in Florida for over 30 years off and on. I visit the beach, go boating and know a lot about Florida's damaging sun and what it takes to stay safe outdoors.
Sunscreen products have expiration dates. Most sunscreen will only last 1-2 years (FDA says at least 3) and then will not be as effective.
Buy sunscreen from a local store so you can see the expiration date for yourself. DO NOT BUY SUNSCREEN ONLINE unless they show or tell you what the expiration date is of the product you will receive. In Florida sunscreen products are for sale everywhere from the grocery store to gas stations.
Apply sunscreen before leaving for the beach and not when you get there. It will have time to sink into the skin. I use Neutrogena SPF 30 on my face - and ears - don't forget the ears. And I wear a hat always, even in the water. The rest of my body gets an SPF 8 or 15 but I never stay long at the beach. For an all day visit, use a higher SPF and re-apply after swimming.
Buy special sensitive skin products for babies and young children.
Don't forget lips. You need a high SPF lip balm.
Worst times of day to be out in the sun is between 10:00AM and 2:00PM, but here in Florida you will burn at almost any time the sun is out.
*Bring along a container of fresh water for rinsing feet, children, faces, whatever. Also bring a lot of drinking water.