Rain is filtered, and has no salt
it has to be the ocean as it is
not plain water
plain water only effects them if they're submerged under it
or not at all im not entirely sure
All water effects them the same.
See Oda's sbs:
Odacchi! Here's a question for you! Crocodile can't fight water because he's "sand," right? Then how does he bathe?! Does he at all?! That's filthy!! Do you bathe, Odacchi? by Crocodile's Mother
O: First of all, let's discuss the problem of Devil Powered folks bathing themselves. People who eat Devil Fruit are "hated" by the sea, and cannot swim. The "sea" here can refer to anything from rivers, pools and baths to any kind of standing water. On a worldwide level, they are all the "sea." When these people enter the water, not only can they not use their powers, they have trouble moving their bodies at all. They might be able to struggle a bit, but it wouldn't do much good. That's if their entire bodies are submerged in the "sea." With less than half the body or just the limbs, it gets easier. Also, rain and dripping water have no effect at all. Therefore, hip baths or showers are the most common choice. Now, in Crocodile's case, "water" is the weakness of his very powers, so his abilities are robbed even in the shower. But it's not like there are enemies around when you take a shower, so I'd bet he would take them even with his powers being blocked. Don't you think? –-And to answer your final question, about myself bathing. As it happens, I do bathe. Twice! In a year! See you next volume!
So it doesn't have to be salt water….it can be any pool of water, but power is drained depending on how much of the body is submerged. So just touching the sea with his hand won't do much.