Open Water Swimming - what is the problem with the speed and movement?

The spikes in the stroke rate are happening every time an auto lap is recorded. This started happening around the middle of February. The pace chart is simply a reflection of the GPS signal quality at the time the watch locks on. I'm guessing that the larger dip in the pace reflects a change of direction? Would you post a screenshot of the track or a link to the activity? The GPS information the watch briefly gets is used to feed the algorithm that actually presents your pace and track. Is the overall pace, distance, and track ok?
Report the stroke rate spiking to Garmin Support (I have reported this already), along with the pace information if you think that’s in error. 

BTW. Envious of your water temp. Here it’s 13C and will drop at least another one or two before it starts tracking up again in September. While there are some here who swim through winter without a wetsuit, I wear one.