Water makes the glue 'fizz', thin & expand more than without, and it will cure quicker, but it will work fine without it. Get the Elmer's ultimate glue at Home Depot or wherever you like to go, as it's the same stuff as Gorilla glue at about 1/2 the price. I've been learning about this stuff as I build my 8' SST out of pink foam, and have found that it's easy to waste glue by adding too much. A little bit goes a long way with this stuff. I have 16 ounces into this plane, and could likely have done it with 8...