Water holds heat far longer than air does, so the Gulf here behaves like a very large storage heater that is always a month behind the calendar.
Spring is the trap. By mid-April the air can be in the high seventies and the beach looks exactly like the brochure, but the sea is still somewhere around seventy, which most people find cold after ten minutes. Children will go in anyway. Adults generally will not.
Late summer is the payoff. August and September deliver the warmest sea of the year, often in the mid eighties, which is bath-like rather than refreshing. By then the school holidays are ending, so the first two weeks of September combine the year's warmest water with a fraction of July's crowd.
October is the window locals guard. Water in the mid seventies, air in the high seventies, the Fishing Rodeo running and the beaches quiet. Snorkelling at the East Pass jetties is at its best now, because summer's boat traffic and wind have dropped and visibility improves.
Winter is for walking, not swimming. From December to March the Gulf sits around sixty to sixty-five. Wetsuits exist, and almost nobody bothers. Santa Rosa Sound and the shallow bay edges warm up a few degrees faster than the open Gulf in spring, which is worth knowing if you are here in April with children.
One practical note: a few days of strong offshore wind can cause upwelling that drops nearshore water temperature sharply and briefly, even in summer. If the sea is unexpectedly cold in July, that is usually why, and it passes.
