U must be allergic to some component present in those waters in which you walked.
Jellyfish. Tiny nearly invisible jellyfish in the water happily swimming about doing their thing looking for other jellyfish to find happiness with and everlasting love.
You don't have hives. You have jellyfish stings.
Sometimes the reactions are so bad you have to be hospitalized.
I'm sorry, but I don't think its jellyfish stings, since where I live, there is barely any at this time. And if there was, why would I be the only one who had stings? Out of the 3 people I was with, I was the only one with this.
ps. my mom told me yesterday it was probably just the cold (since it was not even 10 degres out), she has hives when she's cold.