meadows, dark forest, and plains all 3 have flat land near ocean.
the plains can be troubling as the flying bugs do severe damage to buildings (esp roof tiles and wood bits) and ignore the usual defenses.

you can also just flatten a chunk of land. My harbors are always flattened into the water so I have a place to bust up the ship without losing nails.

Here is something you can think on... I made a very, very large open box (giant U except not rounded) shaped terrain wall and put a stone floor tile 'roof' over a lot of that using ironwood beams. That is the finished basement (the ground is just rock - road type) and port combo, then up the stairs leads into the real castle /fort first floor. Its a Dark forest base, and I cut down a lot of trees and flattened a lot of ground for the work site. The castle part up top is not there yet in this image, its an old/early image.

You can save yourself some trouble: even the extended into the water walls do not stop dwarf swim-around. It takes them a while to get to my boat, but they do get in from time to time.

https://steamcommunity.com/sharedfiles/filedetails/?id=2505240586