Polar bear’s prey, almost humanoid, competent on the water (10)

'competent on the water' is the definition.
(competent like a seaman)

'polar bear's prey almost humanoid' is the wordplay.
'polar bear's prey' becomes 'seal' (polar bears hunt seals).
'almost' means to remove the last letter (most of the word but not all of it).
'humanoid' becomes 'manlike' (like a man).
'seal' with its last letter taken off is 'sea'.
'sea'+'manlike'='SEAMANLIKE'