Would love to help you out but two of your requests don't fit together. Those two are near the pier and reasonable. You see, the closer you get to the waterfront, the less reasonable you are going to find. Seattle is built about 10% on the waterfront and 90% up on a kind of plateau. Down on the waterfront there are really only two hotels (The Waterfront Marriott and the Edgewater). Both are going to run you at least $350 a night. If that's reasonable to you, they should work.
If not, I think you will be OK, almost anywhere in downtown. Seattle is very scooter friendly (my Mom used one for years) so you should have no problem being able to get around town in your scooters. The best way to find a great deal on a hotel is to use Hotwire. Only consider 4 star hotels in the Downtown/Pike Place area of Seattle. We have gotten the Sheraton two or three times for around $100. A great hotel. No matter what you will have to take a taxi to the ship. Only the two aforementioned hotels (that are VERY expensive) are within walking (or scooter) distance.