Resin theory is real, it just got pushed back. I feel like I might have said this to you before, but the bubbles in Sabaody and Fishman Island that allow people to ride around underwater or on the air are made from resin from trees.
Resin is created from plants, and rubber comes from trees, and some resin is created through heat. We've yet to see Luffy use heating techniques like Red Hawk or Red Roc with Gear 5; once Luffy applies heat to his body from those abilities, I think we'll see Luffy produce resin that lets him create the bubbles to fight his enemies underwater.
Hody Jones was defeated by Luffy using a bubble in Fishman Island below the Red Line, I think Luffy will use the same technique, but on himself, to fight Imu underwater (I think Imu has a Mythical Fruit related to the ocean). Ace's Will will let Luffy fight underwater... fire allowing him to fight underwater.
I really loved the resin theory, I hope it still plays out.