The word mere is recorded in Old English as mere ″sea, lake″, corresponding to Old Saxon meri, Old Low Franconian *meri (Dutch meer ″lake, pool″, Picard mer ″pool, lake″, Northern French toponymic element -mer), Old High German mari / meri (German Meer ″sea″), Goth.
The word once included the sea or an arm of the sea in its range of meaning but this marine usage is now obsolete (OED).