Originally you could buy a ticket to go back to Guarma, but Rockstar changed this for whatever reason. While it would be nice to be able to go back and fully explore the area (since we can't do so while shipwrecked there), it's not too much of a bother to me. There aren't that many Guarma exclusive animals with, I think, about 8 in total. Used to be 9 until people found out the Fer-De-Lance snake also spawns in Stillwater Creek. Of these, the only one that's really difficult to complete is the Red-Footed Booby because it only spawns in the air.