What is this man doing?

It turns out a group of men and women swim every day (as far as I can tell) in the harbour just off from Tai Wan Shan Park (大環山公園) between Whampoa Gardens (黃埔花園) and Laguna Verde (海逸豪園). Most of them swim freely without towing a spare tire. This man probably wants to play it safe.

They evidently think the water is clean enough. But I notice that sometimes they have to dive into the water immediately after getting into the water, swim under water to get through a belt of oily garbage, and resurface about 20 meters off the water front. When they finish their swim, the process has to be reversed. They seem to be enjoying themselves, even in winter.

I would not get into the water myself. Would you?