I hate cold water, but I love going to the sea, so I’m obsessed with sea water temperatures of random countries. One thing I noticed is that unlike air temperature where you have “average max, average low”, here you don’t have those, but for example it just says in city xyz the sea water temperature was 24 celsius on july 24. and also 24 celsius day before and after.
Is that the temperature of water during the entire day or an average of some sort? Sea is a huge thing, so I am not sure if the temperature of it changes from morning to afternoon or does it instead keep a stable temperature regardless of time of the day.
I’m going to guess when they refer to the sea temp, they are referring to the part people swim and play in. Which is relatively small compared to the sea.
And I’d also guess the shallow parts of the sea are more susceptible to temperature change through out the day.
I don’t think they take an “average” for the day. Probably the highest, which would be sometime just before sunset. (typically)
Here’s the NOAA National Data Buoy Center. You can click on any buoy and it will give you an hourly history of all sorts of data, including air and water temp. Browsing quickly, it appears water temps don’t vary much within a day - about 1 degree C.
Note that all these buoys are reasonably far out from shore (at least a mile or two). IME, the water right at shore does vary in temp quite a bit, especially on a hot sunny day. The sun can heat up the water quite a few degrees.
The water temperature will exhibit diurnal variation, the way the air temperature changes during the day, but you can expect the variation to be smaller because water holds greater specific heat than air. Wiki link says the surface temperature can vary up to 6 or more degrees in regions with high sun and low winds, though that analysis is more concerned with the ocean than any specific beach. IME, if the water is warm during the day, it’s not going to be freezing cold even if you go swimming before dawn. But, also, in what would count as a comfortable room temperature you have people wearing wetsuits, for the reason that water conducts heat well.
In my experience surfing Southern California beaches, storms and currents can occasionally change the day to day water temp by a few degrees but as a general rule the difference will rarely be more than one degree than the day before. A warm day can make the surface temp quite a lot warmer, but that water never extends more than 10 or 12 inches from the surface - “warm knees, numb feet” is the result for waders and surfers sitting in the lineup.
There’s no appreciable seawater temperature variation over the course of 24 hours due to the huge thermal mass of the oceans. This also applies to the shallows due to the constant tidal turnover with deeper waters.
The most popular time for swimmers to attempt to cross the English Channel (Dover UK to Calais France) is September, as though the warmest months are July and August the warmest sea temperature is during August and September, implying there’s a lag of about a month between changing air temperature and changing sea temperature. The coldest Dover sea is during February and March, and though the air temperature is nice and warm in May the sea temperature during this month is about the same as it is during December.
Where you measure the temperature, and the wind direction is likely a bigger factor than diurnal variation. A few feet out from the beach is going to be warmer than 10 feet down at the end of a pier, let alone at some offshore buoy. Growing up in Chicago, Lake Michigan can be painfully cold even into September when water temps are usually the highest. However, the water was always warmest with a good east wind blowing towards the shore (that’s also the only way to get waves of any appreciable size, so of course it’s one of the rarest experiences in the summer). That caused warmer surface water to “pile up” towards the beach. Winds blowing from the west towards the lake push the warmer water away and cause the colder deep water to come to the surface at the beach. There’s a couple good diagrams here (different directions though) https://magicseaweed.com/news/how-water-temperature-is-impacted-by-wind-and-why-its-important/11088/