I've spoken to a tech from Garmin and apparently the accelerometer is configured to be on the wrist and if placed in a visible place on a board does not work with the same accuracy, hence my pursuit of options where I don't have to stop paddling to monitor data (especially stroke rate, ratio and heart rate).
The Garmins are great kit tho, just not what I need atm.
Further reading into the Coxmate tells how you can adjust the sensitivity of the stroke detection to accomodate different stroke dynamics, ie. rowing (slower) or outrigger/sup (faster). With a 10Hz refresh rate it is also about 10 times faster than the wrist-based Garmins (once per second according to the tech man from Garmin).
Don't get me wrong I'd love a Fenix and have wanted one for a while, but not being able to afford both I think a board mounted GPS suits my training needs better.
And yes I have considered phone app style data but don't really want to stuff around with cases and waterproofing, or have to explain why my work phone has water damage.

Thank you for your replies. I think I'm going to have roll the dice and go with the Coxmate.
Then one day I might get a Garmin too to complete the ensemble!