Question regarding the lagoon at Barra de Potosí

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.