Drop a piece of looted gear on the seabed beside it. It has to be something it could turn into a blueprint, so looted or Gacha-made equipment works and anything you crafted yourself does not. It swims over, grinds the item down, and that is what lets you mount it. Once you are on it thrashes and prompts you for a button. Hit every one; the last prompt dismounts you and feeds it a treat, so carry those yourself. Miss a prompt and it throws you off and turns hostile, costing five per cent of the whole requirement. Keep other creatures well clear, because the thrashing hits very hard.