@lostrack:
I think the feature you are looking for is already there. Under Training Settings>Interval Training, you can set time or distance intervals which will create a lap or split time for each.
Also reference the Run.GPS PDF manual (v2.3.0), section 4.11.
Rowing times are measured in 500m distances. There is an easy way to measure this on land with an ergometer, but not on water.
There is no easy way to do this, but surely it would improve useability to specialise in this. Everything is measured in /500m split times, in rowing. Everything!
MAX split time, avg split would be great.
If there was an advanced feature to use to accelerometer to measure the lurch of the boat and generate a Strokes/min reading, that would be out of this world. (ok, slight melodrama).