Mind game: posit construction of one of these cement canoes with Roman seawater cement (but with topology incorporating hollow cylinders in whatever is designated the stern). Data mining tidbit: 'For underwater structures, lime and volcanic ash were mixed to form mortar, and this mortar and volcanic tuff were packed into wooden forms. The seawater instantly triggered a hot chemical reaction. The lime was hydrated—incorporating water molecules into its structure—and reacted with the ash to cement the whole mixture together.” * Back to the canoe race. Once the flotation test is passed, incorporate sealed mortar canisters into cylinders and when race is underway expose mortar to sea water. Result: jet thrust for a time with residue morphing undetected (maybe) into the original primary construction material. Odds are you'd be disqualified but it might be a fun feasibility study.

*http://www.bloomberg.com/bw/articles/2013-06-14/ancient-roma... ... reaction reportedly reaches temperature approaching 900C.