A marine propeller shaft bearing assembly comprises a bearing (10), for example a polymer bearing, which is preferably divided into three elongate segments (12, 13, 14) and is retained in a housing (4) by a removable key assembly (20). The key assembly is preferably located at the top of the housing and preferably comprises two elongate, tapered keys (21, 22) which extend axially along the housing and cooperate to induce a hoop stress in the bearing. The keys are inserted into the housing and then one or both keys are displaced relative to one another in opposite axial directions into the assembled position. An extraction tool (40, Fig. 5B) may be engaged with the adjacent ends of the keys, and a lubricant may be pumped (60) under pressure between the keys to assist disassembly.