This can be due to a few reasons:
1. The GPS signal being lost and found during an open water and swim speed spikes are created and can affect your Swim TSS
2. Indoors stopping at the wall and reaching for a water bottle, etc can also create speed spikes
Unfortunately, this data cannot be edited like a land activity. However, by deleting the swim TSS score and the Intensity Factor and then hitting 'Apply' your swim TSS will be recalculated based on distance and speed giving you a more accurate score.