This may seem weird but think about it. Kaido could have killed himself just going to ocean. Maybe Kaido has something else than devil fruit.
Maybe he can hide his true power. He could supress his power to make him more vulnerable and his size would decrease and he could take more human like appearance. Normaly/full power he could look like Oars with more inhumane like appearance and increasing his size, comming more beast like. It would explain how he got out from island he crashed to. He would just walk away without seawater doing anything to him. But it is possible too that his crew picked him up. It would also make sense since he tries to kill himself even that he is nearly immortal. He just decreases his strength also looking more like human. Also it would answer why Gekko Moria would want to use Oars' body. He has seen Kaido in his "Oars-form" and tought that he would also be unbeatable if he had gigantic body like Kaido has.
But this is just my crazy idea and I don't expect you agree with this but it could be possible. You never know like when doflamingo turned dressrosa building into strings. But feel free to comment this. Negative and positive thoughts are all welcome. But keep comments in this post, don't start to bully others