My Garmin GPSMAP 556s, which was a standard Swallow Boats option a couple of years ago, is a GPS plotter, electronic compass, SOG log, depthfinder and even fishfinder. It uses quite a lot of current compared to the cheaper monochrome options but even so, it takes a day's concentrated use to wear out a small Yuasa 7ah battery. I now run it off a brick-sized 18ah battery, which lasts about three days if it is run down. However, I keep it topped up with a solar panel, which seems to work even in North Wales.
Navionics charts on an iPad (in a waterproof case or cover) are extremely good, although it can sometimes be difficult to read them from an angle. The charts are also bang up to date, unlike the built-in ones on a plotter. But no depth reading from this solution.
An expensive extra refinement is a Tacktick apparent wind direction and wind speed system. The whirligig wind transducer can be installed on top of the mast and communicates wirelessly with the instrument display down below (mine is on the aft of the centreboard case). No batteries required, as the whole thing is driven by tiny built-in solar panels - one on the transducer and one on the instrument display. Very clever!
