I think the period that this book was illuminated in was from later when really fancy books was more of a business for the church. Usually the very early illuminated books were made by a single monk doing most steps of the process (prepping parchment, calligraphy, painting, binding, etc.) but the later books like this may have had different artists for the illuminated letters vs the borders. So there could have been one monk that just likes sea stuff and he was too good for the other brothers to tell him to stop.

Source: mostly vibes, but I’ve dipped my toes into illumination and guilding