My daugter got the rash as well when we stayed out in Playa Blanca and visited beautiful Barra. She say in the lagoon water. From the internet.....
Is this sea lice or jellyfish? Serious or not?
A rash from jellyfish larvae, also known as seabather's eruption or "sea lice," appears as a red, itchy, bumpy rash, sometimes with blisters, that occurs mostly in areas covered by a bathing suit or hair. It's caused by tiny jellyfish or sea anemone larvae getting trapped against the skin and releasing stinging cells, resulting in an irritating, inflammatory reaction. You can find pictures of this rash, which resembles a severe case of flea or mosquito bites, by searching for "seabather's eruption rash" or "sea lice rash" online.

What it looks like
Red, itchy bumps or blisters: that are raised and can be hard or soft.
Small, clustered red spots, similar to flea bites.
Appearance in specific areas, typically where clothing or hair traps the larvae, such as the groin, back, chest, or under a bathing suit.
Where to find images
Search for "seabather's eruption rash" to find medical illustrations and patient photos showing the rash.
Use terms like "sea lice rash pictures" to see examples.
MedicalNewsToday and CNN offer visual examples alongside descriptions of the rash.

Yep, that is what the bites looked like! However, I didn't have them nearly as bad as some of the poor souls did in many of the photos. And yes, unlike most mosquito bites, they lasted several days as mentioned in the default [All] Google search. Thank you for your help.

Google Images Search for "sea lice rash pictures"

I don't believe the lagoon would have jellyfish. Only the open ocean.

My daugter got the rash as well when we stayed out in Playa Blanca and visited beautiful Barra. She say in the lagoon water. From the internet.....
Is this sea lice or jellyfish? Serious or not?
A rash from jellyfish larvae, also known as seabather's eruption or "sea lice," appears as a red, itchy, bumpy rash, sometimes with blisters, that occurs mostly in areas covered by a bathing suit or hair. It's caused by tiny jellyfish or sea anemone larvae getting trapped against the skin and releasing stinging cells, resulting in an irritating, inflammatory reaction. You can find pictures of this rash, which resembles a severe case of flea or mosquito bites, by searching for "seabather's eruption rash" or "sea lice rash" online.

What it looks like
Red, itchy bumps or blisters: that are raised and can be hard or soft.
Small, clustered red spots, similar to flea bites.
Appearance in specific areas, typically where clothing or hair traps the larvae, such as the groin, back, chest, or under a bathing suit.
Where to find images
Search for "seabather's eruption rash" to find medical illustrations and patient photos showing the rash.
Use terms like "sea lice rash pictures" to see examples.
MedicalNewsToday and CNN offer visual examples alongside descriptions of the rash.

Yep, that is what the bites looked like! However, I didn't have them nearly as bad as some of the poor souls did in many of the photos. And yes, unlike most mosquito bites, they lasted several days as mentioned in the default [All] Google search. Thank you for your help.

Google Images Search for "sea lice rash pictures"

Maybe Scabies.

Many thanks to the Creator of the Universe, I was not infected with those creatures from the nether world! From a million years ago, I am familiar with those evil doers. Primero Dios, never again!

Many thanks to everyone. Thankfully, whatever the little monsters were, the bites are slowly disappearing and the itching is almost gone. Not sure if I am ever going to swim that lagoon again! The ocean is just fine.

Maybe Scabies.

Greetings, Friends. On a recent outing to Barra de Potosí, I decided to swim in the lagoon. The only other time I had done that, the lagoon was open to the ocean. This time, the lagoon was closed to the ocean. There were other folks swimming in the lagoon so my assumption was that it was safe. After spending some time in the water, I felt as if I something was biting me. The bites weren't painful enough to warrant jumping out of the water immediately. Big mistake. Within a day, I was covered with a few dozen bite marks similar to mosquito bites. They itch tremendously. Has anyone ever experienced this or does anyone have any information about what these critters were?

Sounds like the jejenes got you. No-see-ums. Those phuqrs tear me up! It's been extremely odd that we haven't seen any on Playa Principal this year.