• It's unkown what Irkens bathe in but Blue and Zim bathe in paste

However, on Episode 5B "The Wettning Zim discovers that Water (or at least the pollution in the Water) shows a harmful effect on the Irken Skin. Zim would discover that Paste protects the Irken Skin from Water

Invader Blue paste bathing

Invader Blue paste bathing

Zim paste bathing

Zim bathing in paste.