Totally depends which beach you are going to. At hotel beaches dogs are usually not allowed and security will enforce this for safety and sanitary reasons. Go and look for a lonely beach where your dog doesn't bother anybody and there won't be any problems.
i agree. I was going to ask which beach you are going to. Boca Chica, Juan Dolio, Guayacanes all no problem. If you are staying at a resort area tho then i would call the resort and ask them their policies.
I've seen many stray dogs roaming the beach. They love chasing my quad when I'm riding on the beach. There are many quiet beaches that stretches for miles.