den Kopf in ein Buch versenken +++ {accusative}
sich in ihre Arbeit versenken +++ {accusative}
On the other hand:
das Schwert im Meer versenken +++ {dative}
Although these three examples share the same construction and the movement/action of sinking/throwing X into Y is implied, the last one alone requires the dative case for some reason.
I wonder why?