It’s saying that the river only has a rubber band, rather than the person crossing the river. The crocodiles, which we assume are normally present based on the apparent name of said river, are (for some reason) in a meeting, and thus not currently present. Therefore, as the punchline says, you can simply swim across, nothing to worry about (the river current isn’t very strong I guess).
I still don’t get it
It’s saying that the river only has a rubber band, rather than the person crossing the river. The crocodiles, which we assume are normally present based on the apparent name of said river, are (for some reason) in a meeting, and thus not currently present. Therefore, as the punchline says, you can simply swim across, nothing to worry about (the river current isn’t very strong I guess).