Fish Fin?

Fish Fin?

Observation - Fish Fin? - UK and Ireland. Description: In view for about three minute, moving slowly across the water; flapping from side to side, dipping beneath the surface. Light breeze, but whatever it was, it was moving at an angle to the wind rather than with it. It changed direction and moved left to

In view for about three minute, moving slowly across the water; flapping from side to side, dipping beneath the surface. Light breeze, but whatever it was, it was moving at an angle to the wind rather than with it. It changed direction and moved left to right.