イルカが、船の造波抵抗を打ち消す為に、喫水線下の船首に設けた球状船首（バルバス・バウ）で発生した、前方に押し出し出す力を利用して楽に泳ぐ様子です。
イルカは楽をしているというよりかは、楽しんでいるという感じです。
Translated from Japanese by Google
This is how dolphins swim comfortably by using the forward pushing force generated by the bulbous bow (bulbas bow) installed at the bow below the waterline in order to cancel the wave-making resistance of the ship.
The dolphin feels that she is having fun rather than that she is taking it easy.
A dolphin playfully riding the bow wave of a ship
by u/SinjiOnO in oddlysatisfying