Flax (fibres derived from hemp etc) needs the long stems to be retted (ie go rotten ) in water then they are scrutched by a comblike device (this is what is probably driven by the water wheel) to break up the fibres which can then be dried and spun. In the 1840 census you will see most domestic flax spinners around Ballaugh curraghs (the industrial scale mill was at tromode) - I presume the hemp is harvested September (it was in France when I was there) and needs some time to ret - the waterwheels need plenty of water so unlikely to run in summer - I wonder if one years crop retted for a year then was removed to the scrutching mill to make way for the next.
I should imagine that the scrutching mechanism must get clogged regularly and clearing rotating machinery is always dangerous.
James Sampson sounds English (there is a subsequent child Veronica (?if name reflects his losses) in 1769 - is there anything against that entry - his wife Jane moreton (there is also a child in Malew)