They are called Pool Noodles
for kids to play with in a swimming pool. Looks like the foam insulation you get on pipes but comes in bright colours, about 2m long 2 1/2" wide . There was an article in a recent Model Boats on a club in the US that uses them as have-a-go boats.
Simply bend the noodles around a deck/base , glue noodle to itself , waterproof the edges of the already inserted deck into the previously cut slit using acrylic sealant top & bottom , then build superstructure over high coaming on the deck, insert prop, motor etc of your choice. Waterproof wood etc. With the foam tubes acting like the inflatables on a full size boat (all round the boat NOT open at the rear) . Big boat and very seaworthy a la a rubber duck