From what I've seen, the following ports are always bustling with ships:
Scapa Flow (1939-40 only, really)
Loch Ewe (Usually has about 20-30k worth of tonnage, more if 1940 or later)
Boston
NYC
Galveston (again, not a ton, but consistent for the times I've been there)
Belfast
Dover
Those are the ones I've infiltrated the most since starting with GWX. There are various non-major port cities that usually have shipping in them, too. There's one north of Bergen that is undefended and has about 34K tons to sink in it.
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