Yes there is a big danger through shared water. Your horse was exposed to any germ that other horses had, from colds to strangles & etc, by using the communal buckets. If he is vaccinated you've reduced his risk, but I always bring a water bucket to shows. I know enough people that don't "believe" in vaccinating to make it risky.
EIA is the least of your worries re tranmission from a water bucket.
Edit - From this article: http://www.omafra.gov.on.ca/english/...cts/89-114.htm
The EIA virus is spread mechanically. That is, blood, containing the virus has to be transmitted from an infected horse to a susceptible horse by a vector, such as an insect, infected syringes, needles or surgical equipment. The virus can cross the placental barrier and cause fetal infection and a symptomatic stallion can infect healthy mares during breeding."
This is just one way strangles can be transmitted:
"Bacteria can be transmitted through contact with pus or nasal discharges from an infected horse, or from contaminated bedding or barn equipment (water troughs, buckets, etc.)."
"Infection can also be spread via clothing, horse equipment, people, buildings that have recently housed sick horses, vehicles, floats, grooming and veterinary instruments. Anything that comes into contact with an infected horse should be carefully and thoroughly decontaminated. "