I am so sorry you had to go through this experience. How nerve shattering. This certainly does sound like an attack. I am glad your sister was able to emotionally support you. You are fortunate indeed that she cared.

Would you be able to go to a pool or beach in a t-shirt? Is there a comforting object you can have with you (a favourite towel, a book) that you could use to draw your attention to and the anxiety away from you? What about taking baby steps in that you gradually over time get closer and closer to pool or beach. Maybe just park by it the first time, go for a walk the second, dip your toe in the next and so on until finally you are comfortable pool or beach side.

It may comfort you to know that I myself am uncomfortable in a swimsuit in public. It was once excruciating. I've found ways to make it a little easier now.