I have found AIS to the be the tech I'd least like to be without in costal waters, certainly in Northern Europe. The shipping lanes are a serious headache around the English Channel and the North Sea. Being able to see (hopefully) all ships that might be of concern and being able to get their MMSI and give them a call is incredibly handy.
I suppose I'm assuming you have all the standard kit on board (VHF DSC, decent GPS / plotter, (G)EPIRB).
If you're planning round the world, you'll be wanting more interesting radio tech...
Oh, and good batteries and power management is a god send. Without it your power budgets have to have such absurd safety margins you almost always end up needing jerrycans full of diesel everywhere.
Obviously, this being Slashdot, I would have to suggest something like this http://www.inmarsat.com/Services/Maritime/FleetBroadband/SAILOR_250_FleetBroadband.asp