In fallout 3 how do you get on the boat in rivet city?

Next to where the bridge should be on the platform, there is an intercom, just talk into it and they should move the bridge for you, this can be done at the start of the game so you shouldn't have any trouble here.

P.S a guard will also stop you as you cross the bridge to go onto the ship, just tell him you are there to see your dad and he should let you past.