Want to exercise without a mask? Go kayaking.
The mask law applies to indoor and outdoor <public spaces>, but a vessel if privately operated and not rented out / open for hire / taking fare lying passengers / etc, is considered a <private space>.
By law, kayaks are vessels and are thus also private spaces. The grey area is if you are renting a kayak, but then again perhaps you were testing a kayak you wish to later purchase.
Feel free to verify with Marine Department - Harbour Patrol section. They also “friendly advise me” that kayaking is “dangerous” and not to “obstruct” other boats. Never mind jet skis and fishermen...
I would have preferred the following advice:
Wear a life jacket always, equipped with a proper whistle and a strobe light, and wear a hat/helmet mounted light at night.