I would still STRONGLY recommend getting a passport or card. If for some unforseen reason you were to have to fly home from the Bahamas (Family emergency, illness,you got drunk and missed the boat, etc...) you would be "stuck" dealing with the US Goverment.
And they don't seem to want to let us back in the country WITH the passport (if you look at the immigration lines at the airport....)
(And I actually saw "you got drunk and missed the boat" while I was in Port on a RCL cruise. Bubba's ship sailed without him

He was falling down drunk trying to get on OUR ship )