It's salt in general. Water Luffy drank fresh water which has no salt in it. Sea water has salt in it so we can eliminate the common water which leaves a difference of salt. Look at sea prism stones, you can't make water hard, that's just ice and it breaks, to make prison bars or a jutte like Smoker's you could mix salt into the metal. Salt is the real weakness, sea water contains salt, DF users can still bathe.
Oda already addressed this in an SBS as well.
D: 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 Fruit users bathing themselves. People who have eaten a 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.
Plus, in One Piece movie Z we saw Brooke and Luffy weakened by the effects of a spring, which I don't believe has salt in it. So really that means any type of water is a weakness, not just salt water... so naturally salt and fresh water separate would do the same thing.
So bathing does make them weak on varying levels.
Sounds nice enough but then how were Luffy and Boa in her bath during their meeting? How do DF users clean themselves? Why didn't Luffy die after drinking a barrel full of water? How did Momo shower with Robin? How did Absalom stay invisible in the shower with Nami? How does Smoker have water in his Jitte? How are prison bars and handcuffs made with water in them(the heating process to strengthen the material would evaporate water)?
They become weak on varying levels. Oda said they usually take hip-level baths or showers.