Swim lessons.
I was, until quite recently, a terrible swimmer. I had a series of lessons in my youth which helped somewhat. I didn't have any real trouble passing the swim test before I started the open water diver class, but I couldn't do any of the standard strokes and was slow. After a year of diving I decided that this is something I wanted to fix, because with rivers, and currents, and canoes, and the other things I do in the water, sometimes speed makes the difference between being able to get there or not.
I joined Lifetime Fitness and paid for 1:1 swim lessons with one of their coaches last winter. It was not inexpensive. I spent a lot of time in the pool, about 25 hours over a period of several months, including 6 hours with my instructor. At the conclusion of which I swam a 100 yard medley. I'm not going to win any races but I know I can do a sprint across a current now if I need to.
The guy I was taking lessons from is in his late 20s and does this for a living, and he's good at it. In contrast to the college kids some places use, who are trying to make a few bucks over the summer, whose instructional skills are variable, and whose experience is nonexistent. You get what you pay for.
Now, your wife's goals are different, but I think that the same sort of individualized swim instruction would help her, if she's willing to put in the time and money.
