In Puerto Vallarta, you can get by if you just speak English. A lot of the expats who come down here do. Some of them have lived in Puerto Vallarta for 10 to 30 years and they don't really know any Spanish. Because Puerto Vallarta's one of the biggest tourist locations in Mexico, a lot of the Mexicans have learned to speak English.
Even the hotels, resorts, and restaurants will pay for their Mexican workers to take classes for English. I have a friend that lives down the street from me and my Mexican co-worker said, "He definitely lived in Los Angeles. His English is just way too good. There's barely an accent." So I finally asked him. His answer was, "No. I used to work at a timeshare and they paid for my English classes down here at Puerto Vallarta. I just learned English living here in the city."
It's good to be able to speak Spanish. Sometimes when I go to Walmart, all the products are in Spanish. You have to be able to read and understand a bit of Spanish to know what kind of meat you're buying or other similar things.
However, in dealing with the government or for example in getting the cable guy to come out, Spanish is the only language they know. I'm lucky enough because I live a couple of doors down from one of my Mexican co-workers. So if you have a Mexican friend who is always willing to help you, that is definitely helpful. I just had a gas tank delivered so that I would get hot water and gas for my stove to cook with. The delivery guy didn't speak any English whatsoever. I had my Mexican friend who spoke Spanish come so I could make sure I knew that he was installing it right and I knew what I needed to do in case I have a gas leak.
In dealing with government offices in Puerto Vallarta, for example, you need to pay your electric bill or water bill, none of the government staff really speak English. If you go to government-run hospitals, the staff do not really speak English there.
Getting around day-to-day like going down to the boardwalk, buying food, or taking a cab or a bus, you can get by in English; but it is better if you know Spanish.