or maybe it's more likely that England, being much more powerful already than Scandinavian kingdoms in CK2, would also just demolish any Viking invasions on the sea instead of being vulnerable, as they were in real life until the decline of the Viking era, which is entirely undesirable from a gameplay perspective. England is already boring and secure enough as it is, no need to just throw out any foreign threats too
Historically, military ships are expensive endeavors that only very wealthy or very massive states can undertake in a large quantity. Generally, this means the Byzantine empire, the lombards and the arabs. Transport ships are a different matter entirely.
There are ways to prevent rapid naval growth for countries that are not historically strong at sea. With technological disadvantage, financial limitations and limited port size, even a united England would find it hard to fight against Viking ships. And with the recent HOI4 dlc, we know that its possible for naval engagements to not happen even if opposing fleets occupy the same tile/province.
France and the Iberian peninsula should not be able to just wantonly raise up 100+ ships and invade the Byzantines or the arabs so casually. If we really want to be historical, their ships would be no match for the military ships in the eastern mediterranean until much after the Viking age. The crusaders had to request permission to dock at certain ports, the distance between Denmark and England contributed to the end of the North Sea Empire, etc.
Naval power contributes a lot to a country's ability to project strength. Introducing a proper naval combat system would actually put a hamper on a country's growth.
For example, without such a feature, England/Ireland/Iceland can wage unrestricted warfare on any country it wants. They are isolated islands and their ships cannot be sunk, meaning they will always have the capacity to send however many troops their ships can handle that only increases over time. There is no real cooldown time for pillaging because the vikings can raid from England to Georgia. And the historically powerful like the Byzantines and the Arabs are unable to stop naval invasions, making it ridculously difficult to defend their unwieldy large amounts of land.
I'm also not sure if diplomacy is required to cross into other independent fiefdoms (because they should) but if not, then without naval combat, things only get worse.