As a distance swimmer, I had no issue with folding my feet back so as to be more efficient in the water (major stroke flaw when runners start swimming) - and I stand on a balance board to work at a standing desk. But once I started rowing (erg not shell) and needed to fold my feet up towards my kneecaps(!).

I did find I could work on flexibility standing up (no mat needed). I'd just put my foot on the wall, back of my heel on the floor, and gently push forwards. When it got uncomfortable, I'd back off a little bit and hold for a slow count of 15.

Not a trainer, just picked this up from another swimmer who was also using rowing to get through the pandemic pool closures.