Today tried John's way of wearing the watch on my forearm. I'm now sure this is the best option. The watch is easier to see while sailing, the buttons aren't inadvertently activated and the accuracy is really good.
Here's the comparison with the GW52 on my head.
GPSResults V6.147
GW52 ---------------------------------- Watch
2s:
time m [knots] time m [knots]
11:57:41 34.4 33.481+/-0.296 - 11:57:42 34.4 33.465+/-0.456 underhand
12:18:25 34.3 33.372+/-0.161 - 12:18:25 34.3 33.372+/-0.189 underhand
12:03:27 34.0 32.999+/-0.278 - 12:03:28 34.0 33.020+/-0.242 underhand
11:48:17 33.2 32.272+/-0.277 - 11:48:17 33.2 32.254+/-0.383 underhand
12:14:11 33.0 32.110+/-0.211 - 12:14:11 33.1 32.145+/-0.184 underhand
12:10:34 32.4 31.465+/-0.219 - 12:10:34 32.4 31.523+/-0.631 underhand
12:01:05 31.6 30.722+/-0.191 - 11:49:55 31.4 30.557+/-0.516 over hand
11:49:55 31.4 30.559+/-0.402 - 12:01:05 31.4 30.542+/-0.507 over hand
11:35:30 31.3 30.429+/-0.275 - 12:22:08 31.2 30.303+/-0.330 underhand
12:22:08 31.2 30.355+/-0.216 - 11:35:30 31.2 30.291+/-0.255 underhand
10s:
time m [knots]
11:57:43 166.6 32.389+/-0.159 - 11:57:44 166.6 32.376+/-0.221
12:18:27 164.3 31.936+/-0.079 - 12:18:27 164.2 31.916+/-0.122
12:03:31 162.7 31.619+/-0.155 - 12:03:31 162.8 31.652+/-0.177
12:14:16 157.7 30.657+/-0.109 - 12:14:17 157.4 30.592+/-0.148
11:48:20 157.2 30.559+/-0.139 - 11:48:20 157.2 30.554+/-0.159
12:10:37 156.8 30.483+/-0.131 - 12:10:37 156.4 30.397+/-0.268
13:09:56 152.7 29.692+/-0.155 - 13:09:56 152.9 29.723+/-0.150
12:22:13 152.2 29.586+/-0.113 - 12:22:13 152.1 29.563+/-0.167
11:35:32 151.3 29.415+/-0.111 - 11:35:32 151.2 29.384+/-0.112
13:14:52 150.9 29.325+/-0.169 - 13:14:52 150.9 29.341+/-0.159
average[5]: 31.432knots ---------- average[5]: 31.418knots
alpha 500/50m:
time m [knots]
13:22:37 498.1 20.778+/-0.069 - 13:22:37 499.6 20.838+/-0.083
13:29:08 499.4 17.585+/-0.055 - 13:29:08 498.0 17.665+/-0.066
13:25:41 499.4 16.970+/-0.052 - 13:21:20 499.3 16.968+/-0.072
13:21:21 498.7 16.887+/-0.060 - 13:25:41 499.3 16.968+/-0.062
14:14:58 428.5 15.714+/-0.050 - 14:14:58 427.5 15.737+/-0.067
11:36:48 498.7 14.733+/-0.040 - 13:30:46 498.9 15.200+/-0.069
00000000000000000000000000 -- 13:27:18 499.9 14.903+/-0.069
00000000000000000000000000 -- 11:36:48 499.7 14.806+/-0.050
000000000000000000000000000 - 13:11:44 499.2 13.148+/-0.049
000000000000000000000000000 - 11:49:41 426.5 12.996+/-0.059
The watch has extra alphas because min speed is set to 0kts whereas the GW52 is about 3kts
Some of the watch 2s accuracy numbers aren't as good as the GW52, I'll check if there is an under hand, overhand pattern to this. Hmm, well that's interesting only two overhand grips and they are both +/- 0.5 the highest of the ten.
Today's alpha tells the story.

entry leg is overhand, exit leg underhand. Maybe I need to turn the watch upwards a tad.