Is it necessary to hydrate before doing a double red cell blood donation?

Question

So, I gave blood today, as is my habit about every 112 days, going on the apharesis machine to do a double red donation. After the donation, as usual, I found myself having to urinate more often than usual. That's when it struck me. They still give the advice to be well hydrated before donation (and to eat a good meal), but the double red donation has the fluid being taken out replaced by saline solution before it's injected back in. Is drinking a few extra glasses of water before an apharesis donation necessary or even recommended?