I've been meaning to reply to this thread for several days, but did not have time. I see some reference to possibly navigating using base maps instead of marine charts. Based on my experience that would be a big mistake. I have a few pictures that you may find interesting.
First, last season my only real-time electronic navigation device was a little Oregon 400c handheld (although I just bought a GPSmap 640 today). I do not attempt to plot routes with the handheld, but I have plotted routes into numerous marinas and coves on my computer using SeaClear software and downloaded NOAA digital raster charts. I then use software (G7toWin and MapSource) to transfer these routes from my computer to the Oregon.
I get some comfort from plotting these routes on NOAA charts, then transferring the routes to other software and handheld devices, because I get a useful cross-check by overlaying the route plotted using the NOAA charts with the commercial maps provided by Garmin and others. So far, for the limited area where I sail, I've seen good agreement between the NOAA charts, Garmin charts, and my personal observations of shorelines, navaids, and depth soundings. Unfortunately that is not the case with MapSource base maps.
Here, as an example, is the route for a westward exit from my harbor as plotted on the NOAA raster chart with SeaClear. GPS is especially useful here because there are no markers for the channel, and there pretty serious mud flats over near the island:
When I transfer this route into the Oregon, you can see good agreement - the route follows the same relationship to the depth contours (note that North points right in this case):
When I look at this on Google Earth, I also see the route following the same path through the channel:
Finally, here is the exact same route overlayed over the base map that Garmin provided with my MapSource software. Note that your version of MapSource may have a better base map, or even be able to pull the marine charts out of your GPS. Unfortunately, the marine charts in the Oregon 400c cannot be viewed in MapSource, and I did not want to spend the $$$ to buy them, since I am perfectly happy plotting my routes in SeaClear:
As you can see, the MapSource base map shows my route going over land. I have checked the projection and datum for my charts, and have made every adjustment possible to get this fixed.
My basic point is that I would never use base maps to plot a course over water. However, if you have a GPS without marine charts and do not want to buy them, you could use software like SeaClear to plot routes and mark waypoints, obstacles and other POIs, then transfer those over into your handheld GPS to observe your real-time position vs. those POIs. In this case, the POIs would be probably more reliable than anything your base maps show.
Others here have pointed out the risk of dropping a handheld overboard. I alleviated this by buying a $10 bicycle handlebar mount for my GPS and attaching it to the top of my guard rail. That put the screen in a very convenient location that allowed me to glance at it occasionally without taking my eyes too far away from where they should be. I will mount the GPSmap 640 in the same place for next season.