1. Nothing implies that they don't. Artificial Devil Fruits are made using research from Dr. Vegapunk, who studied real Devil Fruits and how they work, meaning he found a way to copy how actual DFs work into artificial fruits. It's more reasonable to assume that they carry the same weaknesses as their foundation is actual DFs.
2. DF users lose the ability to control their powers no matter how much is submerged. They become immobilized at knee deep, meaning, if Ace stuck his pinky into the sea, he can't transform or launch fire, but can still freely move. If the Sunny is in the middle of the ocean and is attacked, due to being in contact with the water, all of its DF powers would be useless.