What's a simple, cheap treatment for my sea lice?

Vinegar. '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.