Much of Oceania is situated within the tropics so there is not a great deal of variation in the sea temperature. The water is always pleasantly warm in Polynesia, Micronesia and Northern Australia (if you dared go in there!).
In the more southerly latitudes of New Zealand and much of Australia seasonal fluctuations are more significant. The sea temperature in the far south of New Zealand is rarely warm and in winter you can really tell the Antarctic isn't too far!