We have done this two years in a row and it is fantastic. You go out on the boat for several hours and follow the dolphins - or rather they choose to stay with you. Captains John & Alma are very knowledgeable about the dolphins and I can thoroughly recommend this excursion. It's not cheap but so worth it, and I think it compares well price wise with other excursions.
I haven't posted about it before because we wanted to keep it to ourselves in case anyone else booked (LOL) but this year we are cruising Eastern so won't be able to do it again. However, next time to Key West, we wouldn't hesitate.