We decided to get it, even though we technically didn't have to. Hannah's dad is deceased (Bryce and I will be married this year) so I'm bringing a copy of his death certificate. Even though we have shared custody of Skyler, we did get the notarized letter from his mom (Skyler shares the Smith-Velasco last name) but we did it just in case. We do not want to have to deal with any issues getting out of Mexico (not really with the cruise aspect but if we had a medical emergency on the ground or something involving the police). This requirement is even included under the passport area even though both parents must apply for the passport together. It's really a matter of protecting children of parental-abduction issues.
So, here's info from the US Goverment Travel website
http://travel.state.gov/travel/cis_pa_tw/cis/cis_970.html:
Minors: Mexican law requires that any non-Mexican citizen under the age of 18 departing Mexico must carry notarized written permission from any parent or guardian not traveling with the child to or from Mexico. This permission must include the name of the parent, the name of the child, the name of anyone traveling with the child, and the notarized signature(s) of the absent parent(s). The State Department recommends that the permission should include travel dates, destinations, airlines and a brief summary of the circumstances surrounding the travel. The child must be carrying the original letter not a facsimile or scanned copy as well as proof of the parent/child relationship (usually a birth certificate or court document) and an original custody decree, if applicable. Travelers should contact the Mexican Embassy or the nearest Mexican consulate for current information.
Hope this was helpful and not alarming.
See you soon!! Lori