Hello,
Since you are having these symptoms only after entering the sea water, so it looks like an allergic reaction to the constituents of sea water like cold water of the ocean or to the inorganic and organic contents of the ocean water. The inorganic contents are the various mineral salts found in the ocean water and the organic contents are the algae, protozoans and the planktons.
Take some antihistaminic like Benadryl and apply calamine lotion. Also avoid the sea water on your face for long and wash your face with fresh water immediately after being in the sea.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
I should have added that the pimples appear about 6 hours after coming out of the ocean, and continue to appear for roughly 18 hours after first appearing.
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