Rain
I live in Houston so our weather is going be similar. I think Sonax PNS is something you might want to look at.
I just corrected my black Acura a month or so ago with BF One Step, then applied PNS based on the strong recommendation from Mike P. I no longer have water spots, it just beads right off and the "self cleaning" is pretty awesome too! Application isn't hard but it's not the easiest, inside a garage, spray a small area, let it set for about 45-60 seconds then wipe/buff. I found that the initial application took some time since it was my first time and the extreme humidity and possibly the heat (95%+ and about 90-95 degrees that day) made it a little harder to set...but man once it set...it looks awesome!
I did my wife's BMW X3 2 weeks ago with BF One Step and PNS, it went a little faster since I knew in's and out's of PNS and was ready for it!
I'd recommend having quite a few high quality MF towels, a good light to inspect it as you go to hit the high spots, work in small areas so it doesn't dry, work in a covered area/garage. If it does dry, just spray a little on the area and wipe, not a big deal!! I also found when you apply it, it tends to be a bit (ok, more than a bit) grabby on the towel. I've since applied BSD from Sonax a couple of weeks after PNS...the car looks better and better...seems like the black gets "deeper"!
Of course there are other products out there but my advice is to take a look as PNS.
Jay