Do it yourself excursions can be a great way to explore ports in Mexico.
It can also be a huge mistake, frustrating and not a very good option.
People who are not intimately familiar with Mexico should not venture off alone.
While you can reach any place you want to visit by Taxi, the trick is to get a Taxi driver away from the port and hopefully finding one that speaks some English. Otherwise he is going to take your wallet for a ride. quite often being white in Mexico means they are going to immediately expect more pesos. True fact, I live in Acapulco.
For a long time i negotiated prices - now i just get in and pay what I am going to pay, I never hear objections for more than a few pesos. But on cruise ship days - the mood changes.
My advice, book excursons with a private agency. When you go to Mazatlan - stop by and see Mazatlan Frank. He is a great guy and very popular.