Daniel R. Collins' citation of the World of Greyhawk Gazeteer is the most appropriate canon source for the climate and weather of Saltmarsh. I would note that what he cites as the "Boxed Set" is more specifically the the World of Greyhawk Glossography World of Greyhawk Glossography (1983) from that set, which "contains a rewrite of David Axler's weather creation article from Dragon".
As far as the citation within U1 that DM's be "guided by any small south-coast English fishing town of the 14th Century", I think the preceding paragraph makes it clear that this is largely for cultural aspects, rather than climate.
Saltmarsh was later placed into the "Viscounty of Salinmoor" by fan work associated with the Living Greyhawk campaign and this was eventually canonized with the publication of the Living Greyhawk Gazeteer (2000). I think the most authoritative source for its climate would come from Gary Holian (lead author on the LGG), in his article "The Kingdom of Keoland" in the Living Greyhawk Journal. Here are some relevant quotes:
The land and its environs are often described as gloomy, helped in large part by the warm mists which waft off the nearby marshes south of Bale Keep and combine with the briny air of the sea to produce a persistent miasma. Fishing, including some whaling, dominates the local economy, which also sports small farms and some cattle grazing...The frequent storms which cross the Azure Sea south of Fairwind Isle to strike the western coast of the Sheldomar Valley often seem to hit this region the hardest.
For a real-world comparison, the humid subtropical climates of any of the US Gulf-coastal states would be good analogs. Snow would be unknown, but the constant high humidity would make winters feel cold to the natives (though not anyone from a more northerly clime). There would likely be a strong rainy season in the summer and dry season in the winter.
https://en.wikipedia.org/wiki/World_of_Greyhawk_Fantasy_Game_Setting#The_Glossography