In looking at something else, suggested by Geo Cur, I came across this nice little picture, indicating a possible mechanism by which Neolithic boatmen might have picked up heavy elongated stones from the sea floor in areas where there are high tides (for example, on the Pembrokeshire coast).
The idea is that you drag your stone down across the mudflats at low tide and mark its position. Then you wait for the tide to rise, position your boat over the stone, and let the falling tide drop your boat down until it is positioned on top of the stone. Then you lash the stone and the boat up together. Then you wait for the tide to rise and lift the boat and the stone -- and with the stone now clear of the bottom, off you paddle in your boat all the way to Somerset, where you deliver your under-slung cargo to your waiting friends who have their rollers and sledges at the ready for the last part of the journey. If you are really smart, you might be able to drop the stone, on a falling tide, directly onto a sledge which has been pre-positioned on the bed of the estuary.
The article in which this idea is proposed has a rather wacky -- and I think unconvincing -- theory that the Breton word "bronbag" (meaning "boat's breast") used for describing megalithic standing stones indicates that big stones were moved in this way by water. I think it much more likely that this word is simply a descriptive one, applied to standing stones which looked, from a distance, like boats positioned on end and bedded into the ground.
All good fun.......
Words as Archaeological Finds
A Further Example of the Ethno-Philological Contribution to the Study of European Megalithism
by Francesco Benozzo, University of Bologna, Dipartimento di Lingue e Letterature Straniere, Via Cartoleria 5, I-40124 Bologna, Italy.
Stout, G. and M. Stout 2008. Newgrange. Cork University Press, Cork.