Kangaroos, when they don’t think they can win a fight on land, will drag their opponent into the water to drown them. They may also retreat into the water in the hopes that their opponent will give up because they do not want to fight in the water.

If their opponent is willing the fight in the water, there is a fair chance a croc will get involved at some point due to the splashing. The roo basically has accepted death and just wants the best chance of their opponent dying if they win or lose.