My fiancé and I actually signed up for the nature tour but it was cancelled (not enough people signed up for it) so we signed up for the Indian River/St. Lucie River cruise.
It was nice -- the captain was informative and entertaining, but living in West Palm Beach, right across from Palm Beach, I see mansions all the time. I wanted to see FL wildlife!
The other thing that got me a little ticked off was that we weren't told that we weren't allowed to bring our own food on board -- we packed a lunch. The captain let us carry it on board and dutifully we didn't eat any of it. My fiancé's guess was that they wanted to be able to sell food and drinks to us. Sure enough, they had a pretty extensive bar, but they only served hot dogs. I like hot dogs, but it didn't compare to the lunch we packed for ourselves. If we were told when we made the reservation that we couldn't bring our own food, that would have been different.
As far as this cruise went though, for the price you can't beat it. It's about $20 per adult for a two and a half hour tour. Last year we rented a boat for half a day with my fiancé's family and our share (including his mother) came to $100. There were seven of us total.
Island Princess also offers other cruises. For more information check out their web site at http://islandprincesscruises.com/