I don’t think it can submerge very far below the surface because in the pictures shown the diesel is still venting to the surface via a pipe.
The specs claim "Depth: approx. 250 m", but maybe it couldn't move much at that depth, just vertically.
Diesel subs can also dive because they have batteries and electric motors.
Military diesel subs have snorkels so they can run their diesels when they are at periscope depth. If they need to go deeper (or don't want a radar-reflecting snorkel sticking up over the surface) they shut down the diesels and rely on batteries. Traditionally huge banks of lead-acid batteries, not sure if newer subs use Li batteries.