Watermelon Help

Does anyone know why this is happening to my watermelons?
They are "Moons and Stars" Heirloom variety, and they are being grown in Jacksonville FL. It is quite hot outside possible that is the reason.
I'm attaching some before and after pictures taken days apart, let me know what you think.

Thanks

Well it appears that the problem may be a calcium deficiency, and that this is blossom end rot.  So at least the problem  isn't too difficult to correct.

you need some dolomitic limestone amendment man...dig the top 2 inches out and put some lime in,and cover with some compost,and water in...

Are these the very first fruits the plant has set?  If so, I've seen some of the heirloom curcubit varieties "abort" the first few fruit - probably because they weren't fertilized.  They usually just yellow and shrivel, so the browning could certainly point to blossom end rot.  Just thought I would throw out another suggestion!

I currently have a big grub infestation that is slowly destroying most of my garden. Unfortunately it is to hot here to use beneficial nematodes at the moment, so i believe that may have something to do with this. Although I am going to apply some calcium.

I've seen a few grubs diggin' in amendments (kelp meal, and such) and I've hand-picked them.. All I can say to do is include small mounds of rocks in your garden as to attract some beneficial snakes, like garters.. They'll eat the hell outta some grubs.

Let us know how it goes.. I applied some calcitic fast acting lime to a few of my ailing pepper plants for instance, and the newest fruits are no longer ailing. Fresh tough skin.

I'm an hour west of Jax.

We aint had no rain. 

Watermelon is a water hog to begin with.  Too little water often presents in fruit as blossom end rot.  The plant is not getting calcium because the soil and roots needs water to move calcium.  The next problem is your soil.  It's sugar sand.  In your area it's a dark grey.  Nutrients flow right through it until the soil is brought into a high condition.  Add lime for now.  Add everything for later.

For the grubs, I've had excellent results with Bt.  Mostly I get coddling moth larvae and leaf rollers.

It's funny you mention bt. I just picked up a bottle of it the other day and it seems to be working. As for the rain that's a problem of it's own.  I'll be updating this when I have some success.

I have had similar problems this year.  I attribute it to pollination.  on one vine, I had 3 melons come in about the same time.  over a couple of weeks, 2 of them at different stages turned black and shriveled, the other one is almost the size of a basketball.  I assume that its not a water/nutrient issue if one wasnt affected, unless the plant somehow knows to abort some fruits and concentrate on one.