You may have to delete and re-upload that activity. I found that issue, and had to delete and re-import. The process to use is:
Import your activity
View it in detail
In the top left corner, switch the activity type from Lap Swimming to Swimming
Click the Edit icon
Change the distance to the correct one
Save
In the top left corner, switch the activity type from Swimming to Lap Swimming
That appeared to work for me just now.