Reading the Navy Lookout article concerning the coupling failure it all starts to make sense. I've seen a number of sea water cooled couplings fail, the usual cause has been a constriction within the cooling pipework usually as a result of internal corrosion of inadequately galvanised components. This leads to a reduced flow and ultimately an over heated bearing/coupling. In a worst case scenario this could lead to a scored shaft and the need to remove the shaft and dress out the damage. The last time I oversaw such a repair the shaft had to go overseas as no UK facility had the infrastructure or experience to carry out the work.