I was checking all my nautical sites, and while on gcaptain.com (I did not recommend because it is slightly technical and focuses more on freighters and cargo ships) andI stumbled across an online AIS mapping system. Wikipedia explains AIS:
The Automatic Identification System (AIS) is an automated tracking system used on ships and by Vessel Traffic Services (VTS) for identifying and locating vessels by electronically exchanging data with other nearby ships and VTS stations. AIS information supplements marine radar, which continues to be the primary method of collision avoidance for water transport.
Information provided by AIS equipment, such as unique identification, position, course, and speed, can be displayed on a screen or an ECDIS. AIS is intended to assist a vessel's watchstanding officers and allow maritime authorities to track and monitor vessel movements. AIS integrates a standardized VHF transceiver with a positioning system such as a LORAN-C or GPS receiver, with other electronic navigation sensors, such as a gyrocompass or rate of turn indicator. Ships outside AIS radio range can be tracked with the Long Range Identification and Tracking (LRIT) system with less frequent transmission.
This site gives a bit of information about vessel movement, and MarineTraffic.com is about what I imagine an air traffic controller looks at all day (when they are awake!). Not only are you able to browse the entire world for AIS reporting ships, but you can view by type of ship, and a few other parameters. Obviously tracking the big yachts is going to be my primary use for this! Just taking a look at some of the yards in Florida, I was able to quickly find Johnny Depp's Boat, VaJoLiRoJa. Fun fact about his boat, if you say the name fast enough, it sounds like "The Jolly Roger", which would make you think of the pirate movies that paid for the boat, but it is actually the first names of all in his family, VAlerie, JOhnnie, LIly-ROse, and JAck.
As you can see on the left side of the screen, I have filtered out everything but the yachts. Also, notice the ad for an iPhone app! That would be a great time killer on the road!
Looks like the Beaver Island ferry, the EMERALD ISLE is a little late for it's 4:30 projected ETA. If you like big boat, or fancy yourself a diehard yacht spotter, this may be the site for you. These closeups only show a few areas in the US, with a lot of commercial boats filtered out, but there are a lot of boats on this site. Be sure to check out some of the destinations around Europe (Med, Monaco, Cannes, etc) to see some of the big boys! Zooming out still gives you a tiling of green squares with the number of reporting vessels in each area, so you can quickly find the hot spots.
I highly recommend you poke around, and if you find any interesting boats, say something in the comments!