If you weren't an athlete prior to SFN and aren't a proficient swimmer, those swim strokes are probably going to aggravate your neck tightness. You could do some water walking if the pool set up for it (no lane lines). There are "pool toys" you can use to do very gentle strength type training. You could also take it easier by grabbing a kick board and doing some laps that way.

I think the key is to take it easy enough to not cause yourself issues, but try to do enough to maintain some of your muscular and cardiovascular strength.