The Rema (YD 116:3) says to eat something and drink something between having fish and meat.
Many have the custom to specifically make a lechaim between fish and meat, meaning they have something alcoholic, like whiskey.
Tosafos (Moed Kattan 11a, brought by Rabbi Akiva Eiger to YD 116:5) say it's dangerous to drink water after fish. Someone told me this is the source for the custom that some who fulfill the Rema specifically have whiskey instead of water. I was wondering if this explanation is brought anywhere.