It depends on what kind of diving you want to do (ie: fresh vs. salt water), what certs you have (OW vs AOW) and how far you're willing to travel. For fresh water, you're going to have to drive a minimum of 1 1/2 to 1 3/4 hr to the closest sites. Also, the number of sites available is somewhat limited right now due to all of the flooding in the Suwannee River area. Ginnie Springs, Blue Grotto, Devils Den, Catfish Hotel (@ Manatee Springs State park), Paradise Spring, Alexander Springs and Blue Springs State Park are pretty much it for right now. GS, BG, DD & Paradise are a minimum of $30 p/p to dive, while the other are less than $10.
If you want salt water, there are 2 main operators that run trips offshore out of Mayport. Atlantic Pro Dive Shop in Jax Beach operates one of the boats and their website is:
Atlantic Pro Divers - Jacksonville #1 Scuba Dive Lesson Shop The other boat is a private operator that works out of Morningstar Marina and his website is:
Offshore Dive Charters I've been on both boats and liked Offshore's better, as it's a bigger boat (smoother ride) & laid out better for diving. There are many artificial reefs/wrecks offshore of Jacksonville but they're located a minimum of 10 miles out so it's a bit of a boat ride to get there vs. the quick 20 minute trips you might be used to in WPB or further south. Nevertheless, there's lots of big fishies up this way and as such, spearfishing is pretty popular. Also, with only a few charter boats, there's not as many divers out there at any given time, unlike south Florida so you'll likely be the only ones at a given dive site. If you can afford it and are certified AOW, I'd recommend giving a trip offshore Jax a try, as it's much different than diving off the west coast where you're located. For exposure this time of year, a 3mm w/hood is a bare minimum but you'll probably do better with 5mm and a hood, as water temps won't get to the 80s for another month yet and they can vary 5-8 degrees in just a few days if the winds switch direction for a while and we get some upwelling of colder ocean water.