Richard J. wrote:

To accommodate tides of 7 metres or more, you would need ramps totalling
90 - 100 metres long (to limit maximum gradient to the DfT guideline
maximum of 1 in 12), and sufficient of them to cope with large passenger
flows. It's not impossible, but it's a significant constraint on the
design, and may limit potential capacity just because the piers would
take up so much room.

Alternatively, vessels could be designed so that they are boarded from a
lower deck when the tide is high, and from an upper deck when the tide
is low. That's how I remember the cross-Channel ferries coped with foot
passengers back in the days of the Hengist and Horsa.

--
John Ray