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If Puma(rock) is light enough to float in water,then can't it be used to make boats?

Puma rock is the name given to the hardened foam of lava when it comes out of a volcano. The inside of a volcano has very high pressure (1600^oc). But when it comes out of the volcano it meets the cool air.

Air and water mixed in the lava bubble out as the pressure falls. The huge difference in temperature gives the lava a cold shock so it freezes immediately. So the bubble gets trapped in the freezing stone. Bubbles make up to 90% of the volume.

Hence pumice is full of air bubbles; it is less dense than water. Hence it floats but as time goes water will enter the bubbles driving out the air. The pumice rock sinks. So it cannot be used to make boats.