What is this??

So, these brown spots multiplied EXTREMELY FAST, and it's got me concerned now...this is one of the best plants I have,
I linked 5 photos...the last 2 MAY be just older/dying leaves, but I added just in case it had anything to do with the main issue here...
If anyone knows what this and/or has a remedy for it, I'm all ears!!

That looks like a fungal disease called "Black spot" and can be eliminated with a fungicide:

https://permies.com/t/93537/toxic-Fungicide#1423936

A really good treatment for powdery mildew is: 1 gal water, 1 T dish soap, 1 T baking soda.

I put it in a spray bottle to make applying it easy.

Some recipes call for adding oil. I have never found a need for the oil.

Oh boy, I really hope so!! It spread so fast...it went from a few leaves, the next day over half the plant!!! I will check out that fungicide and research black spot!! Cross fingers!!

Look up melon anthracnose, a fungal disease caused by the fungus Colletotrichum orbiculare.

1 thing I noticed, when I googled the melon anthracnose, is that, in those pictures, they showed the disease on the underside of the leaf also....on my plant, there is no signs, anywhere on the underside...so would that rule out anthracnose?

Brandon I'm not an expert by any means. Like you I tried to find something that looks like what you have, and came up empty.  My guess is a fungal problem.  What one I'm not sure. The good news is you don't really have to know exactly what it is to get rid of it, though if you are like me, you want to know.  There's several options for a fungicide.  I try to not use anything. I like to let nature balance itself. Unfortunately sometimes if we are going to keep the plants alive we have to intervene.  If I was in this position I might try neem oil.  Mostly because it's so multi functional. It will kill the fungus, add nitrogen, and kill any bugs that might be taking advantage of the weekend state of your plants.  That being said it doesn't discriminate. So I would probably spray it in the evening or very early in the morning.  If you do a light mist it should dry in about 45 min. Once it's dry on the plants it's not supposed to be harmful to the pollinators.  Neem is also I said to to be good for worms, so you don't have to worry about your soil life.  If you decide to go this route, you want to get cold pressed neem oil. It's Superior to the others.
Good luck to you. Let us know how it goes.

Brandon, Black spot is caused by too much water or watering at the wrong time of day.

Does that black "soot" rub off?

No, the black stuff doesn't rub off. I did go ahead and spraya fungicide on the plant, because like I mentioned, in a 24 hour period it took over basically the entire plant that was at ground level....the leaves died over night...I guess the good news is, the fungicide worked, as in, not letting it spread anymore. Honestly I was really not wanting to use anything I can't get from nature, but this is learning period and trying to absorb as much as possible as fast as possible. But, even tho it stopped it, I cut away all the infected leaves, because from what I could tell, it spread SO FAST, that I think it had to be spread by wind....so now, it's basically the vines that started in ground, and whatever was healthy that started climbing up the trellis/fencing I placed....ituse to cover that entire straw covered area, not now....I'm wondering if it will make it

Yea, I tried to look up everything I could...but it seems like it could be anthracnose...and that fungal disease seems to look different on every plant...put symptoms seemed to match for the most part...the fast spread, color (?) Etc

What you have left looks healthy. It will probably bounce back.  
It's frustrating not to know. I have been dealing with a strange thing on one of my tomatoes. It could be lots of things, but doesn't seem exactly one of those things. All we can do is try to figure it out, and do our best and hope it works out.
Good luck, happy gardening.

Yea, it can definitely be frustrating not knowing...hopefully it will still flourish, cross my fingers.
Also, I really appreciate all the help and trying to figure it out...just wanted to reiterate that! I'll definitely let you all know the outcome, for any future reference!!

*UPDATE*

So, my watermelon plant has done very well so far, besides the fact, that it aborts all of the female flowers/fruits. I do have 1 watermelon that has been growing since it started flowering. Does anyone know why the female flower would abort, continously, even if I hand pollinate them? I've included new updated pics!

Hi Brandon. Your vines look great.  I have read stressed vines will drop fruit. To much heat, to little heat, to much water, not enough water, to much fertilizer, not enough fertilizer can all stress out the vine.  Like life I guess it's all about finding that happy medium.  Keep trying. You are ahead of me, my vines are only now starting to grow and look healthy, with no watermelon at all.  A few years ago I bought a watermelon seedling late in the season. Planted it in the ground, and watered it when I thought about it, a lot of times it was when it was wilting, and I thought oh no, better water that poor thing. I got tons of wonderful watermelon.  This year I have several different varieties planted from seed using several different planting methods ( unfortunately I can't plant in the ground because of gophers).   Most of my watermelon vines are sad and not doing much.  I'm not giving up, but I understand your frustration.  Good luck, and thanks for the update.