What is a rift raft?

I once heard in a documentary about steamboats on the Mississippi river that while the wealthy could afford to travel on steamboats, poor people could not and used small rafts that were called "rift...

Rifts are faults where two tectonic plates are diverging from each other. New crust is being created at rift zones.

it's mainly like a inflatable donut you fill with air.

Insufficient information, but at a guess: the displacement?

I don't think there is an antonym for raft. If you tried Dictionary.com, and looked at all the definitions, synonyms and antonyms, and there was no antonym; there probably isn't one.