On board, going to sea? (7,3,5)

'sea?' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'on board going' is the wordplay.
I cannot really understand how this works, but
'board' could be 'plank' (synonyms) and 'plank' is found within the answer.
'going' could be 'walking' (walking is a kind of going) and 'walking' is located in the answer.
The remaining letters 'the' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'to' is the link.

Can you help me to learn more?