It is a spearm whale (Physeter macrocephalus), what is strange.
First of all, you need a big luck to meet a spearm whale in a Sea of Japan, because it is too shallow and innutritious for these animals.
Secondly, spearm whales are not the dolphins, they do not follow a big vessels on root. They do not do it for a good reason: when they dive, they descend abruptly to a depth of 1 km, where they hunt for cephalopods up to an hour. After this kind of exercises average whale just lay on a surface and breath. One can even pat its side at the moment (it is highly unrecommended of cause. Though they are not aggressive, they still can accidentally overthrow the boat on dive). While resting, spearm whale cannot only overtake the BATM, it can`t even evade the incoming vessels, so that cases of a collision with spearm whales is not quite rare.