Ah, I see what the issue is. You’re thinking deepwater ships, whereas the Ironborn primarily use longships which have extremely shallow drafts and a fairly flat bottom.

Unlike a deepwater ship, which can’t really be beached without some very complicated launching procedures, longships can be hauled up off the beach and pushed out to sea very easily, which is part of the reason they were so suited to hit-and-run riverrine missions when you wanted to go a-viking.
So a tidal flat is actually ideal for longboats, as you can sail/row them up pretty close to the tide line, portage them if necessary, and the tide makes launching them again super-smooth. Meanwhile, galleys and other ships with long fin keels and deep drafts can’t pursue the longship onto the tidal flat lest they run around.