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[dimlee]

Temperatures over any land mass, even small one as atoll are higher or lower (at winter in high latitudes) then at open seas. Differences can be surpisingly high at relatively short distances off shore of just 50-100 nautical miles, especially where low or cold currents are.

I mentioned numbers which I remember from my experience, but it would be interesting to read more objective information, certainly. Probably they can be found somewhere in archives of NOAA and of similar organisations. Also in pilot books.
There are also ship log books, however they are not reilable in this respect (except of oceanographic vessels}. Typical thermometer of XX century shipping was of alcohol type, mounted on a bridge wing in a place convenient to take readings but not where it was sufficently protected from direct sunlight and isolated of steel bulkhead (which can heat up to +50C and more in summer).