Merbeing (1204, Orford, East Anglia, England)

One day around 1204, some Orford fishermen caught something unusually heavy in their nets. As they pulled and pulled on the nets in an attempt to get them back on board their boats, and saw what they thought was a large creature tangled up with the rest of their catch. They were extremely surprised when they finally managed to get their catch aboard because there, in the bottom of their boat was a man staring angrily at them. He was described as being naked but with a hairy body; having a long straggly beard and the top of his head being completely bald. Attempts to speak to him failed so the fishermen restrained him and took him back to the town. The ‘merman’ was taken to Orford castle where the castle custodian, Bartholomew de Gladville, kept him prisoner. He and the jailers tried time and time again to question this ‘merman’ but the creature only uttered grunts and strange noises. They noted that when he was fed raw fish he would squeeze the water out of them into his hands and then drink it. Bartholomew de Gladville became frustrated at the creature’s silence and he had the merman tortured by hanging him upside down by his ankles. Despite this ill treatment the merman still did not (or could not?) talk and eventually his jailers gave up. Bartholomew de Gladville then took him to the nearby church but it was obvious that the creature had never seen a church service before either. One day sometime after he was first captured, the merman was taken down to the harbor. Nets had been strung across the entrance and he was set free so that he could enjoy a swim but without escaping. He made straight for the nets and easily escaped under them and headed out to sea, leaping out of the water with joy. Although he spent a little time that day in sight of the harbor, he was never seen again.
Source: 'Humanoid Encounters' by Albert Rosales & http://www.visit-orford.co.uk/articles/the-merman-of-orford

Aquatic Giant (1992, Kavgolov, Leningrad, Russia)

A man named D. Povaliyayev was hang-gliding one day in the early 1990s above the town of Kavgolov in the Leningrad area. The region has many lakes, and in one of these lakes he observed what seemed to be “three gigantic fish”. He descended in order to get a closer view, and was able to observe “swimmers in silvery garb”. He mentioned this episode in a book written by him.
Source: 'Humanoid Encounters' by Albert Rosales

Aquatic Giant (1982, Northern Tian Shan Mountains, Kyrgyzstan)

Either in the Summer of Autumn of 1982, Mark Shteynberg, along with Lt. Colonel Gennady Zverev were conducting periodic training of the reconnaisance divers (also known as frogmen) of the Turkestan and Central Asian military regions. These excercises were taking place in Issyk-Kul, a deep-water lake in the Transiliysk Ala Tau area of Kyrgyzstan. Strangely, a very important official by the name of Major-General V. Demyanko, who was the commander of the Military Diver Service of the Engineer Forces of the Ministry of Defense in the USSR, paid a visit to the officers. Demyanko informed the officers that frogmen on similar training exercises in the Trans-Baikal and West Siberian military regions had encountered 'underwater swimmers' that were human-like but extremely large - being almost three metres in height.
These swimming humanoids were clad in silvery suits that fit tightly around their bodies, and were wearing no equipment or scuba-diving apparatus of any kind despite their depth of fifty metres below the surface. Instead of any human technology, these entities wore sphere-like helmets that concealed their heads. Rightly quite alarmed by these occurences, the local military commander decided to capture one of the aquatic giants. Under the command of an officer, a special group of seven divers were dispatched to carry out this mission. According to the account in which I heard of this anomalous event, the divers apparently were (un)lucky enough to come face to face with one of the humanoids. As the frogmen attempted to capture the giant with a net, the entire group was thrown out of the deep water and up to the surface by some unknown force.
Because the equipment used by the frogmen does not allow surfacing from such depths without the following of the usual decompression process, all the members of the group were struck with aeroebolism (decompression sickness). The only available treatment for this at the time was immediate isolation under decompressive conditions in pressure chambers. Several chambers of this type were in the area, but only one of them was in working condition - and could only contain a maximum of two people. Stupidly, four frogmen were forced into the chamber, and three of them (including the leader of the group) died as a result of this. The rest of them became invalids.
After this event, the previously-mentioned Major-General rushed to Issyk-Kul to warn Shteynberg and Zverev against taking similar actions if the mysterious divers were encountered. Although Issyk-Kul is shallower than the Baikal Lake in which the humanoids were encountered, the depth of the former was apparently deemed sufficient to contain the anomalous beings. The website from which I learnt this story seemed to suggest that the Major-General may have known something that Shteynberg didn't, but this just sounds like needless conspiracy mongering.
It is interesting to note that the staff headquarters of the Turkmenistan Military Region received an order from the Commander-in-Chief of the Land Forces, containing an analysis of the Baikal Lake events and reprimands. This order was supplemented by an information bulletin from the Engineer Forces of the Ministry of Defense headquarters, which listed numerous deepwater lakes in which there had been registered encounters with anomalous things. These things included the appearance of underwater humanoids much like those seen at Baikal, the descent and surfacing of 'giant discs and spheres' (USOs) and powerful luminescence emanating from the deep.
I found this case on two semi-trustworthy UFO websites, and so please take the story with a heaping helping of salt, especially as it seems almost too 'action movie' to be true. If there were actually military deaths caused by aquatic giants, then it would either be so highly-classified that nerdy websites would never know about it, or it would initiate a massive-scale investigation of which the world would have known.
Sources: http://www.ufoinfo.com/news/ruc.shtml and http://ufoupdateslist.com/2003/oct/m26-008.shtml

Merbeing (November 1737, Coast of Exeter, Devon, England)

Fishermen on the shore caught a four-foot-tall humanoid, with duck-like feet and a tail protruding from its back. It tried to escape but was killed when the fishermen beat it with sticks (like humans apparently do whenever they encounter something weird). Another fish-man was caught a few months later in the same area, though this one was described with more seal-like qualities. There are actually a surprisingly large amount of merbeing sightings that were reported in 1737 - and so maybe they were following some kind of a migratory pattern?
Source: 'Humanoid Encounters' by Albert Rosales
Image Credit: http://www.britishmuseum.org/research/collection_online/collection_object_details.aspx?objectId=3370709&partId=1&subject=16589&page=2