Thanks for the comments. We have in fact had 3 regattas so far, and are doing similar routines as paddlerbum is suggesting.
I'm going to suggest very very basic stuff here...
I would suggest stuff like, getting 1/2 the boat to paddle (assuming that 1/2 the boat means more than 8 people at one time) and then getting the other 1/2 to encourage them. For example, front right and back left paddling with front left and back right talking to their partner. This kind of stuff is very encouraging, fun, and yet resistence training.
Tug-of-war...b/c the winning 1/2 has to work together in order to win the competition.
Keep them constantly moving with 1/2 boat going at one time (ie, 20-30 minutes constant with short breaks). For example, front right and back left...fronts or backs only...if you have 10 seats, odds or evens only paddling...
with occasional powers, etc. Keeps their mind in the game b/c they have to be listening...it's resistence training...and they can encourage one another!
Performing race pieces between designated markers and getting the team to try to get further each time in a set time period.
Racing other teams is also good for experience and to keep the motivation high...
That's my two cents, but what do I know!??!