Swimming snake in Tokyo

I promise there is a snake in these photos. I'm sorry it's a bit far away, I was surprised to see a snake swimming in this pond.

It's in a very busy park with lots of children playing, so not really a peaceful place for a snake to find some sun to bask in.

This guys swam around a little bit before heading back to shore. Any idea what it is?

Most common snake in this area is the Japanese rat snake

https://en.m.wikipedia.org/wiki/Japanese_rat_snake

I think this snake is too thick to be a rat snake though.

The most dangerous is the Mamushi https://en.m.wikipedia.org/wiki/Mamushi It's not a very good photo though...