Nautical (at sea) skipping an area that’s crazy (7)

I believe the answer is:
lunatic
'crazy' is the definition.
(lunatic can mean crazy or mad)
'nautical at sea skipping an area' is the wordplay.
I cannot quite see how this works, but
'at' is within the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is found in the answer.
This explanation may well be incorrect...
'that's' is the link.
Can you help me to learn more?
(Other definitions for lunatic that I've seen before include "Type of fringe" , "Foolish person" , "nut" , "bonkers" , "Harebrained" .)
