No good at sea, I'm up to a little over 3mph? (6)

I believe the answer is:

pirate

I'm a little stuck... Click here to teach me more about this clue!

'no good at sea' is the definition.
'pirate' can be an answer for 'sea' (thesaurus). I am not certain of the 'no good at' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for pirate that I've seen before include "Use (another's work) for profit without permission" , "Rat pie (anag)" , "He sailed under the Jolly Roger" , "Buccaneer, sea robber" , "Silver, perhaps" .)

I've seen this clue in The Telegraph.

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