These may be a reaction to the water especially if the beach is not particularly clean. It may be a reaction to the chemical (lotions, sunscreen) you have used it may also be a photosensitivity reaction caused by sun exposure.
Allergic reactions usually present with a rash. A burning itchiness may be present, is this true in your case? Is there any visible rash?
I suggest that you have this assessed by your dermatologist if the itchiness persists. I suggest that you avoid swimming in the same beach and observe any possible triggers for your condition.