I live in Fort Walton beach and have visited Okaloosa Island a bunch of times. It lies between FWB and Destin, connected via a bridge on each side. Okaloosa island has a bunch of public beach access points including the largest in front of what is called 'The Boardwalk' which includes The Crab Trap, Floyds etc and Anglers which opens up onto the pier. Note that the pier costs $1-2 to walk to the end of, which I think is a crock but I hear the fishing is alright. If you want less crowds I'd reccomend moving another 1/2 mile to the east and using Beasley Park to get to the emerald waters. If moderate crowds get to you I'd suggest heading down Santa Rosa Blvd (turn West at the foot of Brooks Bridge) and you'll see 6-7 public beach entry points there as well. Finally, if you want to see more fish I'd reccomend pulling off just before the Destin Bridge and heading to the parking lot ever so slightly to the southwest side of the destin pass. There are a lot more fish and snorkelers and you have a view of the mayhem on crab island as well as the boats coming in and out of the water. You can also follow the coastline to the open ocean for a nice walk. The water is comfortable from April-October although you could go in earlier or later.
If you have a boat take it to Crab Island which is located just beyond the Destin Bridge on a roughly 1 sq mile patch of sandbar. I've seen a lot of people windsurfing in the bay or just sailing or anything else all bayside. There are many entrances to the bay beaches along that side of the road.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.