A Few Days Ago While On Vacation Near Cancun In

A few days ago while on vacation near Cancun in Mexico, I discovered strange red welts and spots on my inner arm and a few patches of discoloration on my upper chest. I was surprised because I don't recall being bitten by anything and suspect I may have contracted some strange fungal infection or been infected by a parasite from swimming in the sea. Like many people at the beach, I climbed onto a big rock that is in the water so I could jump into the sea, and the rock was covered in algae and who knows what else. I also went snorkeling during my trip and was swimming among many fishes. After a couple of days, the red spots turned brown, and small brown spots have appeared on the top of my hand and there are even a few on the top of my other hand. The spots aren't itchy, but they're pretty nasty to look at and I hope that it's nothing serious. I've suffered from eczema off and on throughout my life, but I've never had anything like this. What do you think it could be?