As someone who considers himself as a navigator I must also rank as an ignoramus as the word comes from the Latin for 'We do not know'.
As soon as we get out of site of land, we do not know where we are. We have various techniques at our disposal (GPS aside) that give us a pretty good idea where we might be but we must remember that all our plots and calculations are based on predictions and measurements with varying degrees of accuracy.
The speed of the tidal streams vary from one publication to another and the actual values can be varied by the weather conditions. How do you interpolate between springs and neaps, graphically, how accurate are your plots, by calculation, by eye etc. etc.
Your compass is subject to variation, again a prediction, also subject to annual correction and deviation, a measured correction that if not carried out by a professional compass adjuster is, and I speak from my own experience, very difficult to get accurate. I tend to check the deviation throughout the year when I get the opportunity and constantly update the average but if you look at the raw data it shows a surprisingly wide range.
Boat speed through the water can be gleaned from anything from timing a chip of wood dropped over the side, a spinning paddle wheel under the boat, a towed log impeller to an estimation based on previous experience when a shrimp makes his home in the paddle wheel housing and stops it spinning. Not to mention the changing boat speed during the preceding hour between the log entries!
Then when you add in rough seas, poor visibility, sea sickness it is surprising we ever arrive at our destination!
One way round this is never tell anyone where you are heading (see the top line of the page in the log book that says, “From” - “Towards”), then when you get into a harbour it is just as planned! A better solution, as you should always let someone know your plan, is to work out how long the passage will be so you know the direction of the tidal stream when you arrive. Then instead of heading directly for you destination, set a course for a point a few miles up tide. Then when you sight land and can't spot an identifiable landmark, you will know which direction to turn to head towards your planned destination and you will have the tide in your favour.
Don't say “well I do have a GPS so I WILL know where I am” because I will agree with you but it doesn't make the technique any less useful. There is nothing more frustrating than using your GPS to bring your planned destination right on the nose then to have the wind or engine die and to get swept past on the tide missing last orders in the pub because you are still battling your way back against the tide!