This is my first step be step instructable, and I think it is the first duplo one on this website!

Step 1: What You Need

Duplo pieces:
3 flat 2x4's
15 standard 2x4's
5 standard 2x2's
2 clear 2x2's for windscreen
1 duplo man to drive the boat

3-5 minutes available to put it together.

Step 2: Base, 2 First Layers

Put 6 2x4's upside-down in a row with the long sides touching each other, and one 2x2 at the one end.
Then use the 3 flat 2x4s to connect all the pieces of the layer you already put down.

Step 3: The Deck of the Boat, 3rd Layer

Turn the two layers from the second step right side up.

This step uses 7 of the 2x4s and 2 of the 2x2s.
Look at the pictures to clearly see how to put the layer together.
The opening for the driver should be 2x4 "knobs"
If you have a 2x6 piece available, it would make the strongest stern. (replace it with the 2x2 and 2x4 at the stern)

Step 4: The Engine!

Put on the BIG! outboard engine, made by 2 2x4s and 2 2x2s.
You can also opt for an inboard engine by simply dropping this step :-)

Step 5: Let's Go on Board

Put on the last two pieces, the clear (blueish) 2x2s, and let the fearless driver take his seat.
By putting the windshield one "knob" further forward you could fit two people in the boat, but it would also cause the 2x4s on the side to fall off easily.