Fish hearts, but not hat, could be articles on boats (7)
I believe the answer is:
rudders
'not hat could be articles on boats' is the definition.
I can't tell whether this definition defines the answer.
'fish hearts' is the wordplay.
'fish' becomes 'rudd' (rudd is a kind of fish).
'hearts' becomes 'ers' (I can't justify this - if you can you should give a lot more credence to this answer).
'rudd'+'ers'='RUDDERS'
'but' acts as a link.
Can you help me to learn more?
(Another definition for rudders that I've seen is " Nautical steering mechanisms".)