Sorry, tight for time (working today). My experience is that for the $20, it's worth getting. You can keep 6 lobsters and food fish (grouper, snapper, etc) and a guide did hassle me last year about my paperwork when noticing us fishing without guides. I said of course I had it and he backed off.
Worth getting hold of the customs folks before you leave US, you don't need delays in Nassau and the customs folks in Exuma will probably not be able to help you.
Beautiful place, I hope you have a good trip. Also - don't forget to fish the Rolleville flat near Barraterre at dusk.