Masters swim

I went to my first masters swim yesterday and what a workout it was. I couldn't believe how wrong my swim technique was. Even after one session my swimming improved greatly.

My question is although it's held twice a week I can only make one of them due to other commitments. Is once a week going to be benifcial for my swim indurence and speed, or should I push for twice a week?

I am in winter maintenance so I am on the bike trainer and running but my workouts are not as frequent as summer training.