A map or nav chart of the east Florida coast will show you the northern limits of good (meaning in or near the Gulf Stream) day-boat diving.
From Miami northward, the coast runs North, so the Gulf Stream stays nearby, it's warmer and there's good drift-diving along its western edge near shore. And the reefs also tend to run south-to-north, so you get a free ride along them with the current, your air lasts longer, it's drift-diving, the boat drifts with you.
The northern limit of this semi- tropical paradise is Palm Beach and Jupiter. Above that, you can see the coastline bends back towards the North-Northwest, and the Gulf Stream separates from the near shore. So a dive at, say, Fort Pierce would be a good bit colder, and not so much friendly northward current, than a dive at Jupiter, even though it's "only" 30 miles farther north (acutally NNW). This chart is large-scale, but gives you the idea:
The Gulf of Mexico Loop Current | NOAA CoastWatch & OceanWatch
Another thing--During fall-winter-early spring, the big Canadian cold fronts can do blow through, but surprisingly often they stop expanding and stall out no farther south than right around Fort Pierce, with much warmer air temps south of that. Basically two climates.
I've dived only West Palm and Jupiter, so I guess I'm sort of a hick. But both places have good diving and good "infrastructure" of boats and shops. This is a generalization, but in general West Palm caters more to the novice/intermediate types, while Jupiter is a bit deeper, slightly colder, and is more "adult swim", with some spearos shooting supper. Plenty of underwater creatures, reef sharks who have so far left me alone and let me gawk, sea turtles who are more my speed (literally), moray eels too.
They're both great, winter or summer. And in Feb/March, there is a gathering of Lemon Sharks off Jupiter, you get to watch a school of them pass over you or under you, as you lie quietly at about 70 feet and leave them alone.
