Thank you for your query,
It seems like a viral fever
or the hives are secondary to some allergic reaction
to the beach water or surroundings.
You did not mention the severity of temperature.
During hives, the body temperature is slightly elevated due to vasodilatation and it is not a fever.
can be give if temperature is more than 100 degree F and for hives you can give oral antihistamine in prescribed dose and apply calamine
externally over skin.
If symptoms persisting consult your Doctor.
Hope I have answered your query.