What does "strong swimmer" mean for the Lütschine rafting tour?

Strong swimmer means you are confident and capable in open water, can swim against a mild current, and remain calm when submerged. Comfortable pool swimmers with limited open-water experience should consider this carefully before booking. The Lütschine is a fast-moving glacial river, and in the unlikely event of exiting the boat, you need to be able to manage yourself in moving water until the safety kayak reaches you. Conditions are stricter in early summer when water levels are highest.