Can you teach a Dog to hold its breath underwater?

You can't teach a dog to hold it's breath - it is something it would learn itself.

You can encourage a dog to participate in underwater activities and it will hopefully learn how to hold it's breath with practise, however, no dog is guaranteed to like water.

Some breeds are noted for a general love of water, but even then there are plenty of dogs of that breed that cannot stand water!

My advice would not be to buy a dog just to have fun in the water with it, just in case you are disappointed.

It might not even like boats!

http://www.youtube.com/watch?v=E_jSlrc4JKs

You will have to train it but most water fowl breeds like the labrador retriever, Irish setter, golden retriever etc... can sometimes hold their breath for about 5-9 seconds

That would be instinctive for a short amount of time. Don't think you can "train" them to do it.