It is opening in June and I am curious if anyone can give a better estimate since we are going in June 2011! I think the dive experience is about $60 - I can't remember the exact amount.
In terms of temperature: March is unpredictible for temps, but they will offer you wet suits. We went at the end of April and it was warm but cloudy and chilly at times (coldest day of our whole trip). The water that has animals in it (rays, fish, or dolphins) is quite chilly and the wet suits were welcome. However, if you get cold you can go to the aviary or the lazy river which are both quite warm (really delightfully so). I think you could probably have a great time in march!