Hi Bladerunner,
Destin jetties used to be my favorite place, but that was a long time ago. A very scenic spot. I really don't know what it is like now. The east jetty, just inside the tip used to be the best. It should be a great place for a kayak or similar. Temp in the winter: I thought it was damn cold, probably low 50's at worst, needed a 5 mil suit and a hood to be comfy if it was realy cold. If the weather is warm, it can be much warmer, but I don't have any numbers. We shot a lot of grouper and monster mullet off those jetties. The grouper was in winter.
Don't want to burst your bubble, but I think it is now illegal to spear on the jetties. Check with the Florida Marine Patrol. Just plain freediving should be fine, but the Patrol is pretty sticky about having a dive flag. The vis is usually good on an incoming tide if the wind has not been strong or it hasn't been raining a lot. 15 to 30 ft (or better if you are lucky) should be about normal. Tide is important, so get a tidal current table or find out how long after high tide the current changes. Be careful of the current, the direction probably won't be straight in or out. A strong incoming tide may try to take you into the middle of the cut if you are right at the tip of the jetties.
Good luck
Connor