horses don't have a gag reflux = no throw up capability.
that said - - was your horse eating when he started to spit up water? If so, could be a choke - - if it lasts for more than 1 hour, you might have the vet out to tube/give muscle relaxers (not banamine, though...can't remember name)
If your horse was not eating, the "water" may be drool from a variety of different plants that/ are toxic/bad for horses. What is your horse's environment like?
Also, does he have a temperature?