I always try to carry spare 2m long line in my harness pocket.
In the case I need to tow fellow windsurfer or even retrieve kiters lost board.
Eventually the same line could be used to fix something in my emergency.
I imagine in next few years every windsurfing board ( maybe kite boards too) will be equipped with small watertight compartment that you could have: extra buoyancy equipment, spare parts, two way radio, satellite beacon, some food and drink, small paddle, etc.