First, on a sailboat there are a lot of things that can "fool" the auto focus. It seeks to find something to "fixate" on, so rigging sometimes gets priority instead of you true (usually farther away) objects; and water is particularly tricky since waves are often very "ethereal".

If you have a moving object that is center of frame, moving quickly, etc. it usually does a good job (until it doesn't).

Secondly, my older Nikon digital (like my predecessor F-4 and -5's) allows for the auto focus to be "locked-on" at a point -- by holding down a button -- then adjust the position in the viewfinder to frame the shot, zoom, whatever; and, then to click the shot. That works if you have a moment or wish to dwell on a shot.

My biggest challenge is color balance with digital shots. When I shot slide film, the shots seemed to balance "right out of the bag". It's rare that I don't tinker with a digital image to balance the contrast or color. I know that you can set-up the program on the camera to effect custom balancing -- say to match Kodachrome or Ektacahrome. for example. I've just not spent the time to do that.