No water chestnuts are NOT toxic to dogs. It may have just upset his tummy.
Add one half of a pepsid a/c tablet to wet food twice a day.
If he just has a severe GI upset you can try home care.
He can have Immodium, kayopectate or any OTC at www.wolfdancer.org/health scroll down for products and dosages.Use pepsid a.c for vomiting.
Try a small amount of boiled chicken with all fat drained off, chop meat , or baby food with no onion, beef, turkey, etc add cooked white rice or infant rice cereal.
Go very slow so he does not throw it up.
Dogs get dehydrated and hypoglycemic very easily from vomiting.
Add unflavored pedialyte to his water 50/50 to hydrate him and balance his electrolytes.
Put a drop of pancake syrup on his tongue to raise his blood sugar in case he is a bit hypoglycemic. That should perk him up.
Do not allow him to drink a lot at a time If he throws up water, he can lick ice
Keep him on that very light diet of boiled chicken or baby food for 48 hours after he is back to normal.
Also give him one teaspoon a day of plain yogurt to keep "good" bacteria in his system.
If he develops any abdominal pain, bloating, constipation or cannot keep anything down he needs to be reseen by a vet.
Please let me know how he is doing. I will always be here for you
I hope your baby feels better very soon..