Looking at the major (historical) ports around the North Sea, almost all of them seem to be some distance inland, at least to my knowledge. London, Antwerp, Rotterdam, Amsterdam (which used to require a detour through the Zuiderzee even), and of course Bremen and Hamburg.
I would expect that dealing with the the fluctuations of a river and the depth limits compared to sea would be challenging for a port, especially when compared to to being able to access the sea directly. Yet, most ports directly on the coast that I know of are either primarily fishing harbours or developed in more recent times (like Scheveningen/The Hague, Esbjerg, Folkestone).
So what gives? Was it easier trade with areas further inland, calmer waters, simply more space to dock, all of those at once, or something else?
Thank for a very stimulating question.
Several factors could be involved, the academic literature tend to emphasize the following two aspects:
To give an example, Naismith characterizes the geographical location of London/ Londinium/ Lundenwic (Early Medieval London) as following:
"London occupied the lowest point where the great river could be forded or bridged in pre-modern times (Naismith 2019: 7)."
He further lists the connection where the Thames has at the place where later London develops, namely (marshes), tributaries, and then, drier farmland and woodland. These must have been convenient to provide the city with the necessary living resources like pig ([edited]: for pork) and fodder as well as product for trade. So to speak, London was situated:
"at the center of a web of commerce and rediststribution that pumped supplies in from the sountryside (Naismith 2019: 8)," he summarizes.
The similar circumstances could be also applied to Dorestad, one of the most important trading places (emporium) by the medieval North Sea trading network: it was built on the connection point where the Rhine meets its large trajectory, the Lek, together with other trajectories near there. In other words, Dorestad was the connection point between the maritime trade in the North Sea (like the trade conducted by the Frisians) and waterway transport of the product from the middle- and upstream of the Rhineland like glass of Cologne and wine.
On the other hand, as I underlined before in: Ripa was said to have been one of the biggest markets during the Viking Age, due to easy access from the sea via river......, some ship types prevalent in the early medieval Northern Seas (the North Sea & the Baltic), including the longship as well as the cargo ship of the Vikings, were not so large with shallow draught (up to the 12th century) that they could directly sail into the lower stream of major rivers in North-Western Europe if they wished to. Some trading place on the downstream of river banks by the Northern Seas seemed also to have an outport on the coast, as shown in the following film on the 10th century central place Tissø in Sjælland, now Denmark: https://www.youtube.com/watch?v=BekBcZOiqQE
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