A design is the first step in successful decorating. Keep the design simple, concentrating on the basic theme you select.  A good simple idea, effectively carried out will have the most impact on judges and spectators.

Flags, Balloons, crepe paper, military things, paper mache things, a sign with a name of someone to remember on Memorial Day.

Or, consider the number of crew aboard and make their costuming an integral part of your design.   Costumes can be rented, or made simply of muslin or cotton and painted. What can really work well is to have all members dressed in a single color, white or black, and use accessory items like hats, vests, large ties, belts, etc. ... to match your theme. 

Check your boat's ballast weight when fully decorated and with crew aboard and balance accordingly.

Music, horns, voices