Yes, the Marriott downtown, by the water, is across the street from where the cruise ships leave. The hotel is close to Pike's place, and has wonderful customer service. The lounge has a great happy hour with discounted appetizers, too. I don't think you will be disappointed if you stay there. Have a great time!
The Marriott referenced by previous posters is indeed excellent. It can be pricey, so if it is out of your range and/or is full the night you wish to stay there, the Renaissance downtown is also an excellent property. It is farther away from the waterfront, but is walkable. (A hilly walk, but think of it as burning off the cruise calories pre departure!)
I would recommend you stay at the Seattle Marriott Waterfront at 2100 Alaskan Way. I believe it is right across the street from, or just down the street from the cruise terminals. it's a beautiful hotel right across the street from the water.