Whiteboard
From Openwaterpedia
noun - Whiteboard is a smooth, glossy sheet of white plastic that can be written on with a colored pen or erasable marker in the manner of a blackboard. The white plastic is used by coaches or referees to provide instructions to marathon swimmers during an open water swimming competition or solo channel swim.
The coach wrote '2K to go' on the whiteboard to show his swimmer.
Synonym
chalk board