As I say, the benefit here is mainly to the tourism bureau of the Keys. If they want to fund most of it, bully for them! I can't see moving tourists to the Keys as a national priority, however.
And Amtrak is only for land cruise passengers...
Honestly if there's a transportation need to the Keys and rail is a feasible solution I think the distinction between different classes of passengers is false. Tourists are as stuck in traffic no less than businessmen or students on their way home from college, and their mobility generate economic activity just like commuters or business traffic.
Now, whether the enormous investment a rail line to the Keys would be is warranted or not? I highly doubt that, but not that has in my opinion nothing to do with whether the audience is the "wrong" kind of travellers or not.