Well if he is portrayed as totally immortal he probably doesn't need to eat, sleep, drink or breath. Meaning even if he was a fruit user the water wouldn't do much to him, only take his fruit power. So even if he tries to drown he just ends up living somewhere on the ground of the sea and has to wander around till he gets back up to the surface.
Who said they're fighting? Kaidou landed on the island by chance, it's not like he was looking for them specifically and Kidd has to face Shanks, fighting Kaidou would cost too many men to take out Shanks too.
It's already been said, by a few people sense the chapter came out. I think WB's crew had the Ope Ope and Kadiou kept challenging WB trying to die and WB got annoyed so he had his OOnM user cast immortality on Kaidou so he'd leave him alone.
Why would WB make his beloved son kill himself? Anyways, don't think Kaidou is immortal. He reminds me of Kumadori honestly. It was never said Kaidou cant die. Just he cant be KILLED meaning he can die of natural causes.
I addressed that in another post, the previous user was near death anyway, WB would never have one of his kids sacrifice themselves let alone for that reason. We've just seen Kaidou I'm sure more will be revealed in time, it's not clear yet how or why he can't die. Theoretically if he ate 2 DF's he should die but who knows, there's no way to tell yet.
