I’ve never seen marked lanes in the sea but I have seen areas for swimming that are marked by a rope, buoys or some other barrier. Here is an example, this is what I understand leisure=swimming_area to be:
I haven’t seen this in the UK either but in tourist resorts in the Mediterranean, and in lakes elsewhere. Swimming outside the marked area is often forbidden or discouraged as there may be algae, jet skiers or other hazards.
I don’t think that’s the same as an open-air seawater swimming pool, which would probably be tagged something like leisure=swimming_pool location=outdoor covered=no tidal=yes/no: