Had a cursed idea for the current challenge that I feel won't quite qualify under the rules as it requires an absurdly liberal definition of a "ship" but it did give me an idea for a really out there SB challenge:
The Sea Fort challenge. Draw an offshore structure designed to defend your nation's homeland against an external threat. The Time span for this would be any era which I feel would allow for a wide breadth of entries. Anything from the Solent Forts to Fort Drum to the Texas Towers would qualify.
I think the most important part about the challenges is how they draw people outside their comfort zone to draw something they normally wouldn't. Sea forts are something I've seen very few artists here attempt to draw so this would take that element of the challenge and dial it up a bit.