How Cold?

Andrew Wallace
12.06.19 10:00 AM Comment(s)

We're always being asked how cold the sea is!  In truth you do get to assess it pretty accurately the more you go in the sea, but there are a number of ways you can get a n accurate reading:

1) Use a sports watch

The more expensive sports watches have thermometers built in, eg. the Garmin Fenix 5. These are pretty accurate but will also measure some ambient temperature to some degree.  And, unfortunately, in the case of this model we're finding the thermometer (which is linked to the altimeter and barometer functions) is rather sketchy and we have had to replace it twice! (But that's another story).

Measuring sea temperature when swimming
Sea temperature can give some interesting additional stats to your swim


  • Can be used in the water when swimming.
  • Fairly accurate.
  • Synchs to all of your activities in Garmin etc.


  • Expensive.
  • Prone to break with hard use.

2) Use a water thermometer

Originally designed for testing the temperature of fish tanks and the likes these little gizmos are very good at measuring the sea temp too.  And best of all, they are very cheap!

They come with a sensor on the end of a cable, we just tied a fishing weight to the end and then it sinks nicely, else it tends to float on the surface and then will give an inaccurate reading due to sun/air temperature.

Measureing temperature of the sea for sea swimming
Typical water resistant thermometer


  • Cheap.
  • Easy to use.


  • Can only be used on the shore - the main unit is not waterproof.
  • Will not synch data to your activity on Garmin/Strava etc.