How long is a boat trip from Memphis to Hawaii?

That's quite the trip there. You would be going down the Mississippi River to New Orleans, into the Gulf of Mexico, through the Panama Canal, and then a good distance to Hawaii. This could take a couple of weeks, or longer if you're not in a hurry. Boat speed, idle time for problems, and number of stops/overnights would determine the amount of time it would take. (There could be some waiting time at the Panama Canal, as well.)