There are a ton of pontoon rental places in Destin... this is one I was going to use before I bought my boat.... they even allow dogs on some of the pontoons
http://www.boatrentalsindestin.com/
I think the Pontoons run about $400 - $450 for a whole day (includes the fuel). You can't take the pontoons out in the Gulf... they have to stay in the bay but are good for Crab Island. They rent fishing boats too, which of course can be taken out into the Gulf so you may want to give them a call and see what those run.
You can google "destin pontoon rental" and find a bunch of other places too... you might want to call around and see who's cheapest.
I love going out to Crab Island but my sister (who usually boats with me) hates it... she calls it the big toilet

