Come on Garmin; get it together. Why are swimming and strength workouts the red-headed step children of activity types? From Training Peaks:
Your workouts built with the Structured Workout Builder should sync to your Garmin Connect account. Once your workouts are in Garmin Connect, they should sync to your Garmin device. However, please note, swim workouts and strength workouts are not exportable to Garmin devices at this time. This has to do with a limitation on the Garmin workout screen not working with either of those sport types. The current compatible workout types with the new Garmin Integration are Run, Bike, Cross-train, Mtn Bike, Custom, XC-Ski, Rowing, Other, and Walk.
I'm not sure what the demand for pre-planned cross country ski workouts are, but I have a difficult time believing it's more than swimming or strength training. Looking at the training plans offered by Garmin Connect, I don't see any, but there are a lot that include swimming. And nope - none of those come in either! They show up as a note on your calendar and that's it. My watch supports swimming. Please get the supporting software caught up.