To keep a canoe or a kayak stable even on the ocean while maintaining its original shape since the canoe or kayak is a most unstable boat with the center of gravity biased upward on the water because of its light weight and its boat shape with the round bottom and shallow draft.

A cockpit of a boat hull is provided with a bearing case, and a ballast tank 3 is connected with a rope, pulled up by the rope in water depth where the ballast tank 3 is not in contact with the ground, and stored in the bearing case. The ballast tank 3 is provided with a ballast outlet-inlet and a screw tap 5 so as to be able to put in and out ballast on the spot. A shaft part of the ballast tank 3 is made a float so as to reduce force at the time of pulling up.