A couple of curragh type boats with some planks lashed across them could probably carry a good sized stone.
Thing is though FW that getting the stone onto a boat of some sort or other is probably the easy bit - the hard bit is then crossing pretty dangerous stretches of water to get the stone from A to B. Not saying it wasn't done (the ancients probably knew the waters much better than we do now) and maybe there were times of year when such sea crossings were a doddle.
Pity Pilgrim isn't around - he'd probably be able to shed some light on the technicalities of transporting big stones by sea. Perhaps if the stone was correctly placed it would have acted as a kind of ballast, thereby making the craft more seaworthy.
