What causes tides to be much higher at many seaports at sea?

Answer 1

Much higher than what?

The variation from high to low tide at any given point on the shore is a complicated function of the local topography and ocean currents. Narrow, shallow bays or inlets tend to "funnel" in seawater at high tide: the Bay of Fundy (in Canada) has the most extreme tides in the world, where there can be a difference of over 50 feet between high and low tide.

Large tidal ranges are actually a problem for seaports, because it's difficult to moor boats when there can be several meters difference between high and low tides. The busiest naval port in Europe is Rotterdam, where the tidal range is only around 2.4 meters, and the busiest port in the world (Ningbo-Zhoushan, in China) has a tidal range of well under 2 meters.

There are other considerations as well, of course (access to a deep river or canal system for further transport inland was historically important), so some busy ports do have larger tidal ranges (Singapore and Shanghai have ranges closer to 4 meters), but it's the exception rather than the rule.