A merman has an advantage in fighting in the water and WB cannot fight in the water, so the merman is at a disadvantage, but Kaido fights wherever he is in favor of his opponent.
And the strongest is consolidated, if you are strong on land, kaido will fight with you on land, if you have an advantage in the air, kaido will fight in the air, if you have an advantage at sea, kaido will fight at SEA.