James used transfiguration on his head, hands, and feet to give himself gills to breathe underwater and webbed feet and hands to swim faster. He would've suggested that the Slytherin team stick together to help each other against the merpeople because they can't really send messages to each other underwater when they've found the hostages. He'd help his teammates with their hostages if needed, and he'd want to stick around to make sure his Remus and Abby and Siriusbaby are also found by their respective champions. And he will bite you if you try anything funny, Lestrange
Also Harry is getting a lot of dad-hugs and hair ruffles once they're out of the lake cause he didn't really like having his son tied up at the bottom of a lake, mmk.