It's the larvae of the thimble jellyfish that causes the problem.   Most
likely spots to be badly affected are the neck, behind the ears, underarms
and crotch.  That said, I've seen people that had their core so covered that
it looked like a blanket of psoraisis.

You need to thoroughly wash your wetsuits and bathingsuits after every dive
to make sure that the larvae have been cleaned off of them or, if there are
larvae attached to them, you will be restung or stung again by those when
you re-donn.

Purging your octopus before ascending can help clear out a little surface
area in which you can surface.  Of course, this only helps to a certain
extent and there's no guarantee that the boat will be waiting right in that
spot.

There is some evidence that this stuff actually works:

www.nidaria.com

I recommend application even in areas that will be covered.

Protech Belize used to carry it and may do so now, but they are getting
ready for a location move around that time.  So you might be better off to
try and get a supply of it before you head down.  In the absence of that,
really slather yourself with a greasy sunscreen, especially around the face
and neck.  This helps to keep the larvae from attaching in the first place.
Liberally apply vingegar to the most vulnerable areas after diving or going
for a swim.  Strip out of your wetsuit and bathing suit at the earliest
opportunity.  Thoroughly shower off after every time in the water, but don't
rub yourself as that causes the nematocysts to fire.

If you do find that you've been affected, and it takes a few hours to show
up, gently apply Windex to affected areas and taking Benadryl can help.  I
think liquid Benadryl works better than the pills.

Now, all that said, you may not encounter any at all.  True, it is the
season, but the sea isn't always covered in them.  The DM's don't want to
dive in them either and so will be trying to avoid sites that have blankets
of them laying about.  That said, no quarantee what will have moved over the
site while you are down on your dive.  Don't forget to look up for them.

I've been diving in areas in the spring, that have had thimble jellyfish
action.  While others were, so far, I've never been stung.  Either that or
I'm just not as affected as some others . . .which would surprise me because
of my reaction to mosquito and sandflea bites.