My wife and I both have T-Mobile phones and get excellent reception on the water in and around Seattle, the San Juan islands, and most of the South Sound.
T-Mobile's online coverage map (referenced by oldgrowth) is highly accurate and should let you know how good your coverage will be at your home, business, etc. T-Mobile does NOT attempt to cover every suburban area, so if you live in a rural setting, they may not be the carrier for you.