Errors in geography
Hiding underwater can stop bullets from hitting you. All supersonic bullets (up to .50-caliber) slow dramatically/stop or disintegrate in less than 3 feet (90 cm) of water, but slower velocity bullets, like pistol rounds, need up to 8 feet (2.4 m) of water to slow to non-lethal speeds. (Myth Busters - ep.34)
Soldiers are shown diving in waters north of the Arctic Circle with exposed faces and standard wet suits. Later in the film, characters are shown diving beneath the water without any kind of thermal protection at all. The maximum temperature of the Barents Sea even during the height of summer would require the use of full-body dry suits to avoid dying of hypothermia very quickly.
When Glass meets his XO for the first time, the XO asks 'how was your trip in from Portsmouth?' In the previous scene, we see Glass stalking a stag in Lochaber, Scotland; a navy helo flies in to collect him from there and take him to the USS Arkansas which was at Faslane, also in Scotland. He would not have needed to go via Portsmouth.