'sea lice' are stings from microscopic jelly fish babies.
They can be both painful and itchy. Remove your swimsuit, shower. Afterwards rinse affected areas with dilluted vinegar (about 1 bottle per gallon of water) to deactivate the venom. If still itchy you can use topical anti-itch creams and oral antihistamines. It takes several weeks for this to heal. See your doctor if severe.