[ChristinaM]

In case people don't know, the reason for this is that AIS uses an already existing VHF channel (and on recreational craft sometimes even shares the antenna). AIS is to keep ships from running in to each other. It works great for that. Even on my 34' sailboat I use different means to keep land-based people aware of my location when at sea.

It's pretty easy to see a lot of AIS signals near land using MarineTraffic.com. Since it depends on land-based receivers it's not very up to date sometimes. It's got me 20 miles away right now since there's no closer receiver and that's where I was a few days ago. They do have higher level commercial plans with satellite data access.

What if I need to know about a ship out of line of sight in an emergency at sea? Then I'd call the USCG and they'd check AMVERS for me: https://en.wikipedia.org/wiki/AMVER

Given recent issues with piracy, it's not really a good idea to have a big publicly available database of where relatively slow moving ocean vessels are at a given time. Even if you had all of the world's AIS signals you'd find some gaps in certain parts of the world where vessels switch to receive-only mode.