Two beaches at the same latitude can differ enormously in water temperature, and one of them may be bracing in midsummer. Latitude sets a rough expectation and everything else overrides it.
Currents carry temperature across latitudes
Ocean currents move enormous volumes of water over long distances, and they carry their thermal history with them. A warm current running poleward keeps coasts far north of the tropics unexpectedly mild.
The reverse happens on coasts washed by currents flowing from polar regions. Water arrives cold and stays cold, regardless of how strong the sun is overhead.
This is why western and eastern coasts of the same ocean basin often behave differently. The circulation pattern of the basin, not the sunshine, sets the baseline.
Upwelling brings deep water to the surface
Where wind blows persistently along a coast, surface water is pushed offshore and colder water rises to replace it. That deep water has been isolated from sunlight for a long time.
Upwelling coasts are famously cold to swim in and famously rich in marine life. The same rising water carries nutrients that support dense fisheries and abundant seabirds.
The effect is seasonal and wind driven. A week of the right wind direction can drop the water temperature at a beach sharply while the air stays hot.
Depth and shelf shape matter locally
Shallow water heats quickly because the sun's energy is spread through less volume. A wide, shallow shelf produces noticeably warmer bathing than a coast that drops away immediately.
Enclosed bays amplify this further. Restricted exchange with the open sea lets a bay build up heat through the day and hold it.
Coasts with deep water close inshore never accumulate that heat. The surface layer is continually mixed with cold water from below.
Tides and rivers add short-term swings
On a shallow beach with a large tidal range, water that has crossed sun-warmed sand on a flood tide arrives warmer than the open sea. The same beach is colder on a fresh flood after cool weather.
River outflows change salinity as well as temperature, and fresher water floats on top. Swimmers sometimes feel a warm surface layer over a distinctly cold one.
Rainfall runoff has a similar effect after storms. It also carries whatever the catchment held, which is why bathing advisories often follow heavy rain.
The season lags the sun
Water has a high heat capacity, so the sea warms and cools slowly compared with land. Peak sea temperature arrives well after the longest day, often by a couple of months.
That lag runs the other way in spring. Bright, hot weather early in the season sits above water that is still holding winter conditions.
For a traveller the practical consequence is that late season beats early season for swimming. The air may be cooler and the water will be at its warmest.