I'm on my second year as a certified diver. On my 5th open water dive ever I helped rescue a panic stricken diver in 30ft of water. She survived. On my 15th dive ever we "misplaced" a diver for an hour but he was found about a mile north of us...
Both of those situations sorta tossed me in the deep end out of the gate. I am now motivated to pursue my rescue diver certification and have also learned what not to do from others actions first hand.
Whats it like? Well, its probably easier to image like space. You are out of your element so whatever street cred you have at home is useless. You feel extremely vulnerable at first, especially when you come face to face with sealife as big as you are that has teeth the size of your middle finger. You now realize what it must be like for that little copopod trying to survive in your tank.
That soon gives way to relaxation and excitement. For me this happened after that 5th dive. Knowing that all those little things I took seriously in class actually worked I think made my subconscious a little more comfy. This year I was swimming around upside down, sideways, staying level, and able to work on breathing exercises to extend my bottom time. I was able to focus on marine life. Film some cool stuff. I was not always the first up and out of air. It is spectacular and I plan on getting into re-breathing setups after a while. On one of our dives we dropped two divers off and didn't see them for almost 4 hours. They were just sitting on a ledge watching what comes by in the deep blue for the whole time, must be amazing!
I am what they call a Dramamine diver. I have to take 24h Dramamine the night before and 1 hour before I get to the dock. If I don't, I'm chumming that water. If I do, no problems. If you are on a boat AND you get sick let them know but jump back into the water. The sickness will be gone by the time your feet touch the water.
Is it worth it? Heck yea! Every penny. It may change some opinions you have with keeping saltwater tanks.
Plus, how the heck else are you going to experience stuff like this if you're not certified?
http://www.youtube.com/watch?v=IDVLPSnjjZI
Some advice I got that I believe has helped me out was initially to invest in a few things...
-Classroom and pool dives on the weekends, not vacation. Open water dives on vacation, you get 4 dives and equipment for cheap compared to normal.
-My own mask and snorkel. Price is irrelevant, how it fits is important.
-My own fins. The more relaxed you are the longer your air will last. Having some high end fins that will help you move through the water easier then heavy duty rentals will go a long way in saving air. If you don't want to run out so quick, buy some nice fins. Side effect is you better really be in good shape or you won't be able to walk the day after a long swim.
-My own booties, my own gloves, and my own dive computer. I picked up a Suunto Zoop used in pool training for under $200. This tool isn't required but most all divers will have one. It makes keeping detailed logs and profiles easy plus its nice to dive with two and hopefully the rental gear will come with its own.
-A safety sausage!
-A cheep shorty wetsuit for fishing (snorkeling) afterwards... My routine in Maui is usually... Dive at 5:30am, snack, remove some tan lines, eat a fish taco for lunch, snorkel till 3, margaritas till sunset under palms.
I am in the water probably 5 hours a day.