Hi,
I got a Garmin Fenix2, it records Cadence (strokes/minute).
See above pic of Cadence and speed graphs. (accelerometer in watch records SPM stokes or steps per minute- there's also accelerometer on in heart-rate monitor (but I find the watch accelerometer records better). Had on wrist and on PFD on chest and found better readings on wrist.
Features I use for SUP. Cadence, Speed, Heart-rate, Sunrise/Sunset times, Tide times (via hunting fishing function), Vibrating alarm, for early morning wake-ups without waking the misuss, (better than phone on vibrate). Looks like a normal watch, although it is 13mm high by almost 50mm diameter, it is light.
Also has Multiple world time zones, Temperature, altitude, barometric pressure, vertical speed.
Receives text messages and can be use for live tracking (with iphone 4s and above, bummer I got a iphone 4)
For waterproof case for Iphone 4, Ive been using a Aquapac which has served me well for a few years. Ive used it with my phone on a couple of Downwinders which I spent more time in the water than on the board. Couldn't say if or when it would fail though. From Witworths store