As flip18436572 said, there's really no subsitute for actually getting into the water. Even if you have to drive an hour or so getting in once a week will be much better than any other exercise. That being said there are a number of dryland excercises that competitive swimmers routinely do. Here is a link
http://swimming.about.com/cs/dryland...wim_weight.htm that describes some of them. Most public libraries have, or can get, books on coaching swimming which include dryland exercises.