We first went when my DD was 6 - she was a very strong swimmer and loved everything about DC. Everyone wears a vest or shortie for everything so even not so strong swimmers can do the dolphin swim. They will adapt the program for you too. The last time we went (she was 8 that time), I had recently had surgery so wasn't comfortable doing the actual swim hanging onto the dolphin. So, they just let me have more one-on-one interaction with the dolphin. My DD had not snorkeled before but still loved the reef pool. She hung onto either DH or me and learned to do it while there. She was in heaven the whole time we were there!
I must say when we went last time (she was 8), she freaked a bit at all the rays in the reef pool - she couldn't think of anything other than Crocodile Hunter and no matter how many times I told her the rays barbs were removed, she just couldn't deal with it. We have now gotten her over that fear and she is back to loving snorkeling again.
You will have vests or shorties on all day and you can get noodles for the river so it should be fine. The birds are really great as is all the other stuff.
As others have said, nothing for a 20 month old so if someone will babysit, use that option.