I lack the means to play this game, but this is an interesting character design.
When he swims, his body makes an undulating up and down motion. This means that the main propulsive area is horizontal to the orientation of the body. This type of swimming motion is something that you see only in marine mammals with wide tail flukes….
Ergo…. he’s not propelling himself through the water with his feet, It would only make sense if his “fin skirt” spreads out into a fluke-like surface, that propels him through the water when he dorsoventrally flexes his spine.