The $400 price didn't sound right to me, so I checked
DCL's site. They list the cost of the encounter for adults as $125 and the cost of the swim for adults as $199. I'm not saying that's cheap, but it's certainly better than $400! My DH and DD (8 at the time) did the dolphin swim at Blue Lagoon when we were on the Dream in Feb 2012. They had an absolutely amazing experience. I went along as an observer and get lots of great photos. Here is a link to BL's web site where you can get more info:
http://www.dolphinencounters.com/island-all.php
DH and DD spent lots of time in the water interacting with the dolphins. The highlights were dancing with them and, of course, the foot push at the end. I wasn't sure my DD would go through with it, but she did. We ended up buying the video, and at the end of the foot push you can hear her say "That was AWESOME!" It was definitely a once-in-a-lifetime experience that she will never forget.