Jess, I can pipe in.
I have been diving for years and am a Certified PADI Instructor.
SCUBA is a fantastic sport and I highly recommend it.
There is nothing like diving to make a trip that much better.
As far as using SCUBA to get into shape, don't count on it. Under water you are striving to be relaxed and calm, exertion is actually a bad thing as you use up your air faster and it takes the fun out of diving. I usually "Zen" out as I like to say. I go somewhat into a meditative state and just enjoy.
"If we had known about diving in the 60s, we'd never have needed drugs." - Jerry Garcia
Now, above water you want to be in good shape, that way your cardiovascular system works well and you don't use up air fast. No worries though, anyone of even average health can dive and enjoy it.
A few tips, find a good dive school. Ask the school for references. I expect that of me. PADI is widely accepted but there is NAUI, CMAS, YSCUBA and SSI that are also widely known. Personally, I recommend PADI. They are the largest instructor group[. Go figure.
The advanced classes are very fun and challenging. Once you get your Open Water then you get advanced certified. That doesn't mean you are an advanced diver, it means you are an advanced beginner. Then you can take Rescue. Rescue is probably the funnest class I teach and the funnest class I have taken. You role play emergency scenarios and it is fun as well as a great confidence builder.
PM me if you need any more info or just post here and I will help.
Scooter