CelNav is a subject that is near and dear to my heart. Yes, I have navigated by celnav at sea.
1st, a disclaimer, I had to learn celnav in order to make QM1 (E-6) in the USCG. The problem that I encountered (and so has most every other person), reading the theory does nothing to help you translate theory into the practical art of finding your location. I (and most others), had someone who was knowledgable, walk us through the first 3 fixes and then left us on our own. After you have laid down the tenth fix (or thereabouts), SUDDENLY, the light bulb goes off, and you now understand what all that gobbleygook theory was trying to tell you!!!!!!
I recommend "The American Practical Navigator" by Nathaniel Bowditch as far as trying to learn the theory and the steps required to ascertain a position.
The best way is still to set sail and have someone walk you through the process, the pubs required, and the charting. 20 mile accuracy is acceptable for celnav (I actually maintained a 5 mile accuracy), for sun lines and star plots.
If you learn how to work out a position, be ready to experience a very strong MENTAL orgasm, That's the only way I can describe the feeling that I get when I do a good fix!