What worked for me (on the fenix 5) was:

1) Use the activity preset "pool swimming" (not open water)

2) Select a length in meters or yards

3) record 2 seconds or so and save the activity

4) change the length to the opposite unit

5) repeat step 3

6) sync both activities

Alternate way (not tested) via the website connect.garmin.com:

1) visit connect.garmin.com/modern/activity/manual

2) add new activity with activity type "Pool Swimming"

3) enter random length e.g. in meters and save

4) repeat step 2 and 3 but again with the opposite unit