Re: Speed/Depth/Wind conundrum
I'd go for a small chartplotter/gps, sounder as a minimum.. but I really like through-the-water boat speed indication so that a) your sail trim results are clear and b) you can better interpret your SOG from the GPS wrt to the currents working with/against you.
As Jon says, todays' plotters are 'magic', one of the features I find most interesting is the ability to 'see' progress (or, sadly, the lack of it) as the GPS reels off Distance Made Good, or Estimated Time Remaining etc.. Move the cursor and know exactly how far that distant landmark, island, point, buoy is off... neat stuff.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)