Thank you to Christian, author of the RTL-SDR AIS Android App for letting us know that he's updated his app and it now includes a waterfall display for tuning the AIS frequency. Tuning the AIS frequency is not required on higher end RTL-SDR dongles that come with a TCXO (Temperature Compensated Oscillator), but cheaper RTL-SDRs will have significant frequency offsets that will require the offset to be determined after a few minutes of warm up time. The easiest way to do this is with a waterfall display as that allows you to tune the frequency manually.
AIS stands for Automatic Identification System and is used by ships to broadcast their GPS locations in order to help avoid collisions and aide with rescues. An RTL-SDR with the right software can be used to receive and decode these signals, and plot ship positions on a map.