Find as much money as you can.

The escaping thief is losing money all over the lake. Hurry to find it or the money will sink!
Make money out of beer mats, writing 1, 5, 10, 100 and 1000 on them with a waterproof marker.
Drop the beer mats all over the lake in the water.
The rowing crew tries to pick up as many as they can.

Use balloons with notes attached.