en kyst = a coast, kysten = the coast, kyster = coasts, kysterne = the coasts
Or do you wonder why anyone would swim along more than one coast? They're probably ultra-endurance swimmers ;-)
They are baywatchers on a tour
Usually people swim along one coast at a time, so, it just sounds odd to say the coasts.
Maybe you and I are swimming along two different coasts?
The y in kysterne sounds more like an ø to me. Is it really the case?