My wife dives. We both got certified together, became instructors together, did our cave training together. It's great, but it gets really pricey when everything you buy is x2!
Underwater disagreements can also be interesting!
Like TSandM said, be careful what you wish for! Actually, it's great that we both dive. My wife isn't quite adventurous as I am (she would use a different word), but we've worked through that. She realizes that one day I will probably go on a Doria trip. She has no desire to do that.
My wife and I do a couple of vacations each year with family. My sister dives, but she has one of those nondiving spouses. My mother, also a nondiver, comes along on those vacations. (We live across the country from each other and decided destination vacations were a good way to see each other and the world). What helps is that we have more than one of each - diver and nondiver. We plan combo vacations all the time. Last year we spent a few days in Mexico. The nondivers spent time on the beach while the divers went diving. We did lots of shore diving and one tank trips so we got to spend SIs together and the nondivers loved the beach time. In a couple of weeks we'll be in S. Florida for a week. We have 2 tank trips scheduled a couple of mornings while the nondivers eat breakfast and shop. We have an afternoon 2 tank trip scheduled while the nondivers shop and site see. Out of 5 days the divers are spending 12 hours diving (6 dives) off boats and the rest of the diving will happen from shore while the nondivers are on the beach. Then we'll head up to N. Florida and dive the springs while the nondivers enjoy the state parks.
It can be tough balancing out the activities, but it is possible. Your best bet would be to find another disfunctional
couple so you have a dive buddy and your gf has a surface buddy. Good luck!