An adjustable swivel base assembly and track for swivel mounting fishing boat downrigger or planar board masts, including a planar mounting base plate, a pair of stationary riser supports on the base plate, and a planar bottom bearing plate stationarily supported on the upper ends of the supports. Vertically spaced planar bottom and top bearings are stationarily affixed to the bearing plate and a planar swivel top plate is supported on and sandwiched slidably between the bearings, and is adapted to carry and swivel the mast load. An indexable locking system selectively locks the swivel plate in any one of a plurality of angularly spaced positions. A track is mounted on a fishing boat and has side edge flanges with mutually facing in-turned lips that overlap opposite side edges of the base plate when slidably and releasably mounted in the track.