At the time the DLC is set the Norse settlers of the regions corresponding to in-game Laithlind had not yet been integrated into the Kingdom of Norway (though I'm not sure they were united as a single entity either).
So something odd and hilarious has been happening during some of my naval battles...
Eventually they plummet back down into the sea and fall unconscious.
Probably just because they never conquered anything on the British Isles, and that's the main focus of the DLC. Sure, we have Frisia too, but they're basically a Danish faction.
Yeah from my playtesting it seems that sending Egil away was the cause of the problem. Had no problems if I kept him in the party. Would it perhaps be possible to disable sending companions away for story mode? It seems there's a few places in the plot where it has been an issue for people.