Haquoi, if it helps, mymidwife and my personal trainer both strongly advocate swimming as the water helps keeps your temperature down (apparently temperature regulation is important in the first 12 weeks). You may of course have an underlying health condition that led your midwife to advise you as she did, but if you are (so far as you know) generally healthy, then swimming is fine!
I'm a rower and I've not been in a boat or on an erg for 5 weeks, it's torture and I've lost 6 pounds (muscle softening into fat
). Swimming and walking is keeping me sane until I feel up to getting back in the boat (going for a paddle this evening, eek, but will be a gentle one as I'm taking beginners out).