Shoggoth2588 said:
...back on topic though: I can't recall any games that are about Dark Ages naval combat.
That's because there's nothing to base an idea on and most likely was exceptionally rare.
Sea control throughout the Middle Ages was virtually nonexistent and the sea was bigger, faster and safer (but more expensive) highway to march armies on than land, especially if you were attack coastal areas because you could land and attack without warning - the MO of the likes of the Norse and the Muslims of the time. Men could attack any point of land that wasn't already manned, reinforced and defended at will, and if need be, get back on their ships and escape just as easily because no one had any idea of how to intercept ships by guess work and most lacked the numbers of ships to make any possible naval battle anything more than a fight with a handful of ships on either side, which the Norse Mount and Blade DLC does as best as can be done.
What combat you'd face would be naval combat but a aquatic extension of land warfare using ships to surround and cut off raiders from escaping while your army wiped the raiders out on land.
This is what Alfred the Great tried to do against the Vikings and met with mixed success, building large, slow, high draft galleys to trap Viking longships in the landing beach if his patrols failed to defeat the raiders on land. What happened in what little can be documented in pretty much only one surviving testament of a battle is that his men chased the Vikings to their ships, which Alfred's own ships couldn't engage because of the shallow waters they were in, killed some, tried to board and capture some of the longships before they could escape into water too deep to wade through, which when they did were too quick for his large slow galley to range in more than a passing ranged attack of throwing spears before the longships got out of reach and slipped into the open sea.
A more effective counter to the Viking raids was devised by Alfred's daughter, Æthelflæd, which largely revolved around planting large wooden stakes in vulnerable shallow areas to prevent the Vikings from landing on them or going up river and instead landing father away from opportune targets thus allowing the Anglo-Saxons more time to be warned, muster and counter attack.
Hardly something to build a game around.
Keep the context in mind, raiding parties of, at most, hundreds of men in a dozen of less ships, not thousands of ships in hundreds of ships, making lightning quick strikes to get in, rape and pillage, and get out as quickly as possible. There was no way for the Anglo-Saxons to stop the Great Heathan Army at sea because they didn't know it was coming and they didn't have the ships to stop it if they did, nor was the invasion point largely surmised and opposed as it was at the Battle of Sluys in 1340, by which point Medieval men had at least formed a rudimentary idea of how to fight war at sea.