Honestly, yes that is a bad idea. First thing is you dont know the language. Second it is a foreign country, I actually think that some people dont realize that when travelling in Mexico bc some places in MX dont seem like it. Third the roads in MX are very unpredictable. There is not signage like there is in the states.
Also just to add to it, the Policia in Mexico are not kind. They do not mess around, if you get pulled over you need to be prepared to possibly go to jail. Which means you could miss the boat.
There are tons of guides when you get off the ship. Just rent one for the day, less hassle and a lot safer all around.