You need other opinions too, but here's mine. We headed to Atlantis and spent the whole day there, so it was actually a longer day for us than Castaway day. There are pools everywhere on the grounds, but the biggest area is the pyramid. They have racing slides off the back side, a huge plunge side down the front, and a tube ride through the pyramid that ends with you in a plexiglass tunnel that goes through a shark lagoon. There are a few other slides sprinkled here and there; we really enjoyed a pull and drop side on the other side of the property. The property is really big so you can do some walking. Several pools have grottos and waterfalls that can be fun for kids (and dads) to play in. The beach is right there too, although we didn't do that. We also got my daughter's hair braided there, and avoided other busier places. (it was clean and they used our comb). It's not a full blown water park though, but if you're water people you can kill the day easily.
You got the money part right --- pay for a room at Comfort, and that gets you into Atlantis. The only other money we spent was for lunch and the hairbraiding; the Dig was "included" so you might wander through it again at that price.
I know less about the Dolphin Encounter than the Dolphin swim. I'm torn on this --- it cost a ton and I'd not do it again, but we have some priceless pictures and my daughter had a great time. It was more of a photo shoot with dolphins, than a lot of interaction with them; we'd have preferred the latter. I'm sure others will provide details on the Encounter. The dolphin place is right there at Blue Lagoon, so you can walk over and enjoy the lagoon after your encounter, and take a later boat back --- but you need to be aware of the boat shuttle schedules because you're going "off tour" at that point.
Happy hunting!
