Across which to ship a lot of water? (5)

I believe the answer is:

ocean

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

'a lot of water?' is the definition.
(oceans are made up of water)

I can't explain the remainder of the clue.

(Other definitions for ocean that I've seen before include "Pacific, perhaps" , "One of the Seven Seas" , "Very large area of sea" , "Large expanse of water" , "Extensive sea area" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app