Boats (Bts) in this period were generally oared craft. At the stop, an oared boat can be turned 180 by rowing with one side only, or even more so by rowing normally on one side and backwatering with the other side.
Even an Olympic-level eight only manages just over 12 knots at full speed. A longship is estimated to have had a top speed of 10 knots - even full-on galleys are usually only estimated to have managed no more than 11 knots or so. Most ancient and medieval Bts wouldn't have managed anything like that speed. If you watch a rowing race, once the crews stop rowing, the boat stops pdq. If need be, unlike a modern naval vessel, an oared boat can be brought to a rapid stop by using the oars as brakes.
Of course, I've not tried either pivoting or using the oars as brakes myself with anything larger than a Dragon-boat on the Thames... A longship is a fairly different category of vessel to a Dragon-boat or a rowing-boat.
FWIW in this era, other than individuals' shooting bows etc., combat between riverine vessels would have had to involve one crew boarding the other (rams and catapults being confined to larger, sea-going, vessels) so full-on naval combat would generally have to have effectively been conducted with the vessels stationary relative to each other (at least long enough for the boarding party to cross from boat to boat).
Tim Child