We really enjoyed the tour of the ship. Be sure and get the audio handset included with the entrance fee. It gives you so much more information and history than just the displays on the ship. Read the displays too though because a lot of the displays tell things that the audio tour doesn't.
The museum is small but interesting (and included in the reasonable cost), and if your ancestors entered the US via Galveston, you can do a computer search of the records if all those who entered here.