The connection between creatures such as the Loch Ness Monster and the semi-mythical Water Horses of Highland folklore has long been discussed and held to be a continuous theme by various crypto-researchers (myself included).
A new source of such stories has now been made available online and free to the public by the University of Edinburgh in the form of the Carmichael Watson Project. Carmichael Watson wrote the well known work on Highland culture and folklore entitled Carmina Gadelica published in 1900 but this is a mere fraction of his total research which is now available online here.
The problem is most of what I looked at had not been translated from the Gaelic, so in that sense it is still a work in progress. Nevertheless, entering the search term "water-horse" yielded 14 results which gave stories from the isle of Lewis and Harris and no doubt elsewhere but without their translation, I cannot be sure. I have contacted them asking for clarification on future translations.