Marine creature in position, almost adjacent to bay? (3-5)

'marine creature' is the definition.

'position almost adjacent to bay?' is the wordplay.
'position' becomes 'seat' (seating is a kind of positioning).
'almost' means to remove the last letter (most of the word but not all of it).
'adjacent to' means one lot of letters go next to another.
'bay?' becomes 'horse' (bay is a kind of horse**).
'seat' with its final letter taken away is 'sea'.
'sea'+'horse'='SEA-HORSE'

'in' acts as a link.