Skin rash after swimming (Myrtle Beach: top, warm, water)

[Frozen]

Was down in first part of October
the ocean water was still warm ( at least to me ) about 75%
swam for 2 hours a day for the week we were there
but every night at bed time the top of my feet broke out in a rash
small itchy bumps that lasted about a week after we left Myrtle Beach

This happens every time I come down, doesn't happen if I'm in the pool, only in the ocean.
any ideas?

Thanks
slightly thawed