Wouldn’t an EMP disable any devices any electronic devices the passengers carried as well?
Well that depends on the size and strength of the EMP. You might want to think of this like a lightning strike. Although they are very different, some of the same principals apply. Now the bigger, most conductive, structures will be the most vulnerable. Power lines and circuit boards are perfect targets for these weapons. In the case of the Carnival Splendor the switchboards and the diesel generators would be the first casualties along with the ships power lines. If these items absorb most or all of the EMP then items such as cell phones may survive because they are small. If the force of the EMP is not mostly absorbed by these items then smaller conductors may be affected. In this way it is like lightning as the current hits the largest conductors and excess current will find smaller conductors. Now this is a simplistic look at a complex situation, but it gives you the general idea.