It depends a lot, of course, on the style of map you are going for. I'll link you to some of my maps which feature harbors.

Haibianr: Hand-drawn, scanned, colored in PS. The docks aren't really to-scale. Picture 32 of 37 from My maps

Haibianr docks: Digitally created in PS. This one is a more 'realistic' style and I reckon the docks are pretty well in scale.
Picture 36 of 37 from My maps

Tarina: Created by mushing up and painting over satellite imagery. I guess it's the most realistic of all, since it's based on real-world locations in and around Italy.
Picture 20 of 37 from My maps

As you can see, the styles are very different. If I can give you any specific tips please let me know.
M