I was curious so looked up daily coastal sea temperature for Fiji and Hawaii. The Hawaii data was better but both appear consistent. It looks like daily water temperatures fluctuate about 1-2 C max per day at a coastal locations. Out on the reefs may fluctuate somewhat less. Over a week or a month the temperatures seem to be in a 2-3 C degree range (weather most likely accounting for the wider range). Over a year almost 5 C degrees from lowest to highest. None of these changes would be sudden, not more than 2 degrees C per day. Looking at Hawaiian weather charts for the last month along with sea temperature it looks like they experienced a 2 degree drop in 12 hours along with a barometric low, indicating a storm or bad weather. Air temperature dropped by almost 7 degrees C (to a chilly 72F ) in the same period.