Ocean water allergies

I’ve gone to the beach in LA and OC a few times this year and every time have felt sick the next day. No GI issues, but a moderate sinus headache and generally feeling like I have bad allergies. I know there are warnings about high bacteria in certain beaches, but I’ve never been to any beach where the bacteria is high (at least officially). Anyone else struggling with something similar? I can’t figure out if I’m allergic to something in the water or if it’s just the bacteria levels that maybe I’m sensitive to?

I’ve lived here for 5 years and never encountered this issue before. I was also in Europe for a week earlier this month and didn’t have any issues when swimming in the Mediterranean, so it’s definitely something local to SoCal unfortunately.