This view is based upon folks travelling without a car who simply wish to visit Amsterdam as a tourist for a day.
If you really want to see Dutch capitol I wouldn't advise going on the day trip excursion with DFDS.
We have just returned from a 2 day trip with all of 3 hours to yourself in Amsterdam city.
You might work out your "city visit time" by the time the boat arrives and leaves - this is NOT the case.
DFDS Ferry was 1 hour late into the Dutch terminal after 15 hours at sea, and generally is late according to other passengers who travel regularly.
After late arrival it takes about 1 hour for the transfer coaches to fill up.
It then takes 45-55minutes into the centre of Amsterdam by coach, ; if you've got on the free city centre tour it adds another hour.
You are requested to be at the pick-up point in central Amsterdam for the return journey about 2 hours proir to departure.
This gives you between 3 and 5 hours in Amsterdam before you have to head back for your 15 hour boat ride. Not good.
If you have a decent cabin instead of slumming it down below and you also take into account what you will spend on food and drink on board (which was fairly pricey) you can take the 1hr 40min flight and easily overnight in a decent 3 or 4 star hotel and see the city for a full day for the same if not cheaper price.
This trip is for boat enthusiasts not city visitors.