https://phys.org/news/2018-07-algae-blooms-poland-baltic-sea.html

Emergency water rescuers told vacationers on hot sandy beaches—from Swinoujscie in the west to Gdynia in the east—not to enter the sea, where thick green-brown cyanobacteria colonies have grown and pose a health threat.

Regional sanitation authorities have issued warnings that contact with the bacteria may cause allergies and rashes. Drinking contaminated water can also lead to serious digestive problems.