Wikipedia
Sinkbox
A sinkbox is a specialized hunting blind used by waterfowl hunters. It consists of a weighted, partially submerged enclosure large enough to hold one or more hunters and suspended from a floating platform. It is placed into calm water so that the hunter may wait with the waterline at approximately shoulder height.
Sinkboxes were typically used by market (rather than sport) hunters for duck or other waterfowl in both the United States and Canada.