I rescued a DD when he was about 5 years old. He was scared to death of water. He loved to retrieve tennis balls. I took him with my other DD to the local lake and walked in with him. If he wanted to play he had to join us. He came right in and we played. We didn't do retrieves or train, we played. Now I can't keep him out of water.
If it is too cold for you to walk in, get a second dog and tie yours up on the bank. Make her watch the other dog have fun. Once it looks like she can't take it any more, send her for one. Quit before she is done so she will want more next time. Make it fun and not training.