This is nothing new to SCUBA divers, but the ability to take notes or draw diagrams in the water is quite useful when doing open water swimming coaching.
We use this writing slate and pencil from Mares, primarily designed for SCUBA diving, simple but highly effective.
This is us taking notes on a student's open water swimming technique when we were out on the sea swimming route at Freshwater East.
It helps us to remember what we want to point out in the session write up and also can be used to draw diagrams to help the student understand things like body positioning, arms or leg positioning/angles and so on.
