How comfortable in the water is she? Does she go swimming just for fun? Do you two ever go to a pool and just dive to the bottom for fun? Throw stuff in and retrieve them? It seems to me that a lot of people get into watersports who aren't really all that comfortable with the water. I suggest doing something fun in the water, and not necessarily diving specific.
I took up diving before I met my husband, but I learned to surf from him. It was not a fun process, and I generally find it more fun to surf without him than with him. There's more pressure, he likes bigger waves, we just aren't at the same level with surfing. Even with diving, he's faster, and I like to poke along looking at fishes. Cold definitely affects me more too. I don't think I'll ever dive anything colder than 65.
We have a fun factor ratio for our water sports - the colder it is, the more fun it had better be. My bar is probably set higher than his.