[below + deck + -s1]-s is a native English suffix used in the formation of adverbs. Other words that usethe affix -s include: always, betimes, needs, unawares
Examples of 'belowdecks' in a sentence
belowdecks
Allen grabbed an overhead rail and swung into the companionway leading belowdecks, then turned back to Dieter.
Terman, Douglas CORMORANT
There is a dive centre belowdecks with a decompression chamber and storage space for aviation fuel.