Hi Tom, and welcome. Sorry for the delay in answering you, it's a busy time of year in Florida for gardeners. This is the height of our growing season here (and covering up plants to protect from the cold takes time, too).
I'm right with you on the cucumbers and melons. Also squash, both summer and winter varieties (which are related to the melons and cukes) suffer the same fate. With me, it's mostly been either powdery mildew (a huge headache in humid weather) or pickerel worms which eat the leaves, and there's another type of caterpillar that burrows into the stems of the plants. I've tried several times to grow cukes, melons and squash, and had them all die before any harvest, so I've given up. Lots of other things work better here.
Bt is the recommended spray for preventing the depradations of the caterpillars, and it is organic approved (not toxic to you). You can buy it as a concentrate at any garden center.
Spraying the leaves with a solution of baking soda about 1/2tsp to a quart of water can prevent the fungal attack but you need to be very diligent, spraying at least every second day to keep it on the leaf surfaces. It changes the pH of the leaf so it is not a welcoming host for the fungal spores. Watering carefully, or with micro-irrigation to keep the leaves dry can help, and it's extremely important to always water in the early morning so the leaves can dry off by nightfall. Leaving moisture on the leaves over night is an invitation to fungal attack. (of course you can't help it if it rains . . )
Another thing you might have going on is nematodes in the soil. If you built your raised bed on top of an area where there was lawn or grasses growing, it's very likely that nematodes are starting to invade your bed. You can tell if you have these nasty little microscopic worms if you have pulled up any plants and seen nodules that look like beads or knots on the roots. Peppers and tomatoes are very susceptible, but a lot of veggies can be affected. To rid your soil of nematodes (which exist naturally in the sandy soil here in Florida, sadly) You will need to solarize your soil for about 6 weeks - during the hottest, driest part of the year is best, late may into June. Here's an article from University of Florida on the how and why.
You also need to be aware of the seasonal preferences of the various veggies. This is a great time of year for planting your cool-season stuff like the leafy green veg. Spinach, lettuce, and all the cabbage relatives like broccoli (which does GREAT, btw) love this weather. Going forward into spring, you can plant beans, tomatoes and peppers around the end of February when we won't expect too many more cold nights like this week.
Amending the soil is an ongoing process here because the high heat of summer, and heavy rains leach a lot of the good organic materials, and they also break down and are used up by the plants faster here. You can't add enough organic materials, more is always better! Good compost is available for free from most landfills here, but you have to go and get it, load and unload yourself. I love to amend my raised bed with alfalfa pellets - it's horse food, available in a 50lb. bag for about $16 at feed stores and places like Tractor Supply. For your 10 x 10 bed, I'd be adding a whole bag of pellets at least once a year. Water well and dig them in. You'll get a little bit of a stable-y smell for a day or two while they break up. If you get some, be sure it's pure alfalfa, with no additives or vitamins in it. (they may not be harmful but will cost more and not help your plants)