In the next sketch the situation is sketched what happens at a groyne if this one is more or less above the waterlevel.
The current alongshore the coast, for example a tidal current or a winddriven longshore current, is pushed into the sea at point A. At the end of the groyne the current will bend around it and go bach to the beach (B). The current to the beach will split up in a reverse current (C) and a longshore current (D). If the longshore current is opposite the picture will be mirrored. In cas of an opposite tidal and wind-driven longshore current there will be a lot of turbulence at the end of the groyne.
At higher waterlevels if the groyne will be submerged completely the currents will slowdown and disappear. However, groynes are covered with shells and other sharp objects. So it is easy to get hurt.
The currents A, B and C are always there at fixed moments depending on the tidal curve and winddriven current moments. Even if no breaking waves are there.
The currents are good predictable, however there is only 1 swimming advise:
STAY AWAY FROM THESE OBSTACLES. At least 50 m and with strong alongshore currents do not swim at all over there.