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).

Advantages:

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

Disadvantages:

• Expensive.
• Prone to break with hard use.
  

2) Use a water thermometer

Advantages:

• Cheap.
• Easy to use.

Disadvantages:

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