I think you have gotten a lot of good advice. I will just throw my 2 cents in as someone who was recently in your shoes. We were big snorkelers and took many trips over the past 5-6 years to go snorkeling. My Dad had started diving about 5 years ago and was always trying to get me to try it. We had many opportunities to do it as we were in good snorkeling locations (Cozumel, Belize, Roatan, Grand Cayman). However, I was a bit apprehensive as to if I would like it or not. We booked a snorkel trip to Bonaire about a year in advance. As I was reading and doing more and more research I thought that it would be a shame to make the investment to travel somewhere so geared towards diving and not at least see if we liked it. I already had a relationship with the LDS as a serious snorkeler they had sold me personal gear (snorkel, mask, fins, accessories) before and so we took a Discover Scuba in the pool for $25 for the two of us. It was about 20 minutes in the class room and about 20-30 mins in the pool. I was so worried I would feel uncomfortable or just not be able to clear my ears, etc. I am not sure if it was all the snorkeling we had done or what, but as soon as the reg was in my mouth it was like second nature to me. At that point because I knew we loved snorkeling (seeing cool stuff underwater) and I though that I could handle at least trying SCUBA, we decided to save our pennies for an open water class. We are also a young couple who have to watch our money closely and honestly the expense of diving (as opposed to the expense of snorkeling) had also turned us off before. But we signed up for classroom sessions and did that portion and then waited another month or two until we could afford it and did our open water dives locally. Then when we were on our trip we had ALOT of flexibility and at least in Bonaire where it is very affordable to dive we saved a lot of time and money being ready to do some diving from boats, some with guides and some on our own. I am not sure what experience level is necessary to dive CR and Nic or what the diving situation is there, but if you think you will truly love diving, and you are already making a large investment for the trip I would encourage you to try a pool discover at home and then if you love it try to make the OW work before you go. If you still aren't sure maybe try a discover at a well researched dive provider on the trip. I would just hate for you to get down there, love the diving and then feel like you have to shell out a lot of time and money to keep doing it (studying on vacation) while on your trip. I would also hate for you to have a bad experience. Building a good relationship with you dive instructor was really important for me. I don't want to be coddled or ignored, but I want to be trained well so that I can take care of myself and my buddy to the best of my ability. Obviously I want to and will take more training and trips, but I am VERY glad I did my OW before our trip.
I am sure you will have a fabulous trip either way and I hope you post a trip report and let us know how it goes!