The links between the U series and Greyhawk, at least within the series itself, are so anemic as to be nearly non-existent. As far as I can tell, these are all the things that would place them in Greyhawk:
1. When the town of Saltmarsh is described, the book gives the DM a number of parameters to use when building their own Saltmarsh - basically, asking where's the inn, who's in charge, who do they worship, and so on. At the end, it says "On the World of Greyhawk map, Saltmarsh is located here, and there are two larger towns about 20 miles away named Burle and Seaton."
2. At some points in the adventure, books by Nystul and Tenser show up, but play little role other than as loot.
3. In U3, there's an imprisoned NPC who's sent from the Hold of the Sea Princes to investigate things.
That's pretty much all there is to it. The effort it would take to place this in Forgotten Realms, Eberron, or whatever homebrew setting you have that doesn't deviate too far from the D&D norm is pretty close to nil.