good post mechta which again points out it what i have written before : strange this piece kept buoyant crossing the entire indian ocean !
the specific density of all the materials used are heavier than water so only air inside will prevent it from sinking . and as you pointed out : the structure will not be build fully air / watertight to prevent pressure cycling.
it might be than an airpocket kept it afloat , but that sounds incredible assuming it floated about a year being rocked by the waves and drifted across the ocean.