Wikivoyage says of Clifton Beach:
This was the world's most popular silver-sand beach and health resort throughout the 20th century, but it was affected by an oil spill in 2003 when the Greek registered oil tanker MV Tasman Spirit ran aground and over 12,000 tons of oil spilled into the Arabian Sea in what is considered by some to be the largest environmental disaster in Pakistan's history, so it is better take a stroll rather than swim in the sea because of polluted sea water and strong currents.
I suspect the info might be out of date. 14 years after the spill, is it still a reason to avoid swimming there?
Besides the spill, how bad are currents and other circumstances that may affect tourist swimmers?