Do you mean: "Go swimming" by that ?
If so, the North Sea not exactly gets warm (especially by June). A trip to some place in Southern Europe would be recommended for that.
Otherwise, I agree with Chootie's recommendation about going to The Netherlands using either rail or car. No point in flying CGN
(usually also quite costly).
Alternative could be to fly to the U.K. and stay on the South Coast (Brighton and the likes via LGW
) or the Coast in East Anglia (Harwich, Clacton, ... via STN
Expensive to stay (in busy summer).
Expensive to get to unless you fly in, as the german railway has a monopoly for access (cars/autos get carried on freight cars across the causeway linking Sylt with the mainland).
It is a lenghty drive from Cologne to Sylt along Motorway A1 ! Lots of construction between Münster all the way to Hamburg, with frequent closures during weekends between Bremen and Hamburg. And you will have to pass the tunnel under the river Elbe in Hamburg which is notoriously riddled by traffic files during rush-hours and vacationing traffic at weekend when tubes get closed for refurbishment.
Carpe diem ! Life is too short to waste your time ! Keep in mind, that today is the first day of the rest of your life !