It's a copper punt, also sometimes called a balsa raft. Found this description on the internet:
A copper punt was a small raft, often, if not usually, made of balsa wood, in the form of a catamaran, little more than six feet long and about four feet in beam. Originally it had been used by the shipwright, in the days of wooden ships with copper sheathing, which covered the underwater parts of the ship to just above the waterline. After a storm, it was quite usual to find much of the copper peeled off at the waterline, and the shipwright would launch the copper punt, and would go round the waterline, his mouth full of copper tacks, hammering the copper sheets back into place. In the steel navy, the copper punt remained, and was used by the painter to go round the waterline 'cutting-in' the boot-topping, i.e., painting the nice straight line that marks the change from the ship's side grey to the underwater anti-fouling paint. An example of a copper punt can be seen on board HMS Warrior in Portsmouth.
Copper punts were carried on some major RN warships at least up until WWII.