This video, titled "Hatsujima｜Diving with Dolphins｜January 21, 2020" (初島 イルカとダイビング　２０２０年１月２１日), was released by "Sazanami Marine Diving School" (サザナミマリンダイビングスクール).

Hatsujima, located in Atami, has been a place where dolphins have been swimming since ancient times.
Because of this, the possibility of encountering dolphins swimming around Hatsujima is incredibly high, even when diving.
This video shows a dolphin encounter during a dive.
The video shows two dolphins swimming around people in a friendly manner. They're so adorable!

In addition to the dolphins, the video also shows a school of labracoglossa (sea chubs).
Hatsujima is a resort island close to the Tokyo metropolitan area where you can spend a luxurious time surrounded by the sea, which is so blue it's known as "Hatsujima Blue."