So sorry you're going through this.
*Definitely wear a tight rash guard.
*I was surprised that pee and/or ammonia (applied in the shower and rinsed off) seemed to help.
*Take off your bathing suit before rinsing your body in fresh water
*There is a product called Safesea that protects against jellyfish stings and sea lice which I apply under suit before swimming (barn door for you but for future reference)
Hope you're feeling better soon.