GPS as a technology is incredibly accurate and precise. The problem in narrow waterways is that even with the modern chartplotters of today, the fix lags behind – sometimes even twenty seconds or so. GPS is a very important aid in determining the rough position of a vessel, but the micro-scale navigation is just so much easier to do with navigational aids (buoys, lighthouses, glitters, leading marks, etc.)

> But doesn't virtually anyone have a smartphone now?

Perhaps one tool amongst the others, but they have several disadvantages

* You have to hold them -> one hand less available to anything useful

* Even if you can figure out a way to mount a phone so that you can read it (remember to somehow accommodate charging!), the screen is still quite small -> can't be viewed at a distance

Even if you somehow get all of the physical problems sorted, software problems still persist:

* Where are you going to get your charts from? Certainly do not expect to not have to pay for them (a couple of hundred of dollars is not uncommon)

* How are you going to view them? The solutions of the big players are quite expensive (couple hundred quid per season). Sure, the play store and such are full of some random dude's chart viewers, but are you going to trust that some random ass guy got the projection and datum conversions right? Even if they mostly did, even an error of ten meters or so could be critical.

tl;dr: a good phone-based solution costs at least as much as a basic chartplotter. In addition, you have to make multiple compromises.