Concept 3 Rower vs Swimming

First, thank you!  And stay safe!

The erg will work your lats which may be useful for swimming, but otherwise the motion is completely different.  (It is excellent general exercise though.)  If there is any way you can get some swim cords, that's your best dry-land alternative to swimming.

I double the post above that the exercises are very different, although that's not to say time spent on the erg is useless.  It's a great full-body workout if you row using correct form and it strengthens your core muscles, which is important to training in all disciplines.

How long have you been swimming?  Personally, I swam competitively every summer until I graduated from the league (at 18) but I rowed on my high school team.  That said, I was able to get back in the pool every May and it only took a couple days for me to "adapt" my body to swimming from my months spent rowing (both on water and on the erg).  However, if you're relatively new to swimming and form is a big concern, I would expect it to take a little longer.

I can not see how you can use one as a substitute for the other. But that dose not mean you should not use the C2! In my opinion the C2 is probably the best piece of fitness equipment that exists. Keep training on it and you will build a VERY strong base, and once you have that run and biking will be easy. Unfortunately the only way you can improve you swim technique is in the pool.