Geophysical Research Letters
Seawater temperatures have been measured at United States tide gauges throughout most of the 20th century. All available records have been digitized, and the longest 14 have been analyzed by linear least-square regression. The largest positive trend is from Boston MA (+3.6 ± 0.4°C per century), and the largest negative trend is at Charleston SC (-0.1 ± 0.3°C per century). No consistent latitudinal or eastcoast vs. west-coast patterns are discernable, but air temperature trends are typically greater than seawater changes.
Maul, George A.; Davis, Andria M.; and Simmons, Jeffrey W., "Seawater Temperature Trends at USA Tide Gauge Sites" (2001). Ocean Engineering and Marine Sciences Faculty Publications. 28.