Ramp and deck built around a plastic water trough. All wood is reclaimed from shipping palettes sourced for free. Plastic spigot added to trough, and a ledge dropped in near the surface to help the ducks enter and exit.
The ramp was initially a bit narrower and steeper, but one of our ducks is clumsier than the other and he seems to appreciate the extra width and more gradual angle. Water (and other duck-related substances) can be drained via the spigot, but the trough is easily removed for more complete cleaning. Fully portable: just drain the water and the trough and ramp both lift out.
It took a bit of coaxing for the ducks to figure out how to climb up the ramp, but they seem to be enjoying themselves now!
Total cost, under $50.
Note: the plastic of the tub was a bit thicker where the spigot attaches, so I had to carve off a few millimeters to expose enough threads for a solid connection.
Update, March 3, 2013:
~7 months after construction, everything is holding up well: the wood has aged gracefully. The "step" inside is no longer needed because our ducks have mastered their entries and exits, but apart from that we haven't needed to make any modifications.
We drain the water every 1-3 days, and nothing makes a duck happier than a fresh pond! I found that the spigot is usually clogged with... silt... by the time we want to empty it, but there's no need to reach in to clear it when you want to drain: lifting up the tub an inch or two and then letting it drop back onto its base is usually all it takes to get the flow started.
An alternative to the spigot: a large bathtub plug would work here too, but that would require either reaching into the blecchy water to pull it, or extending a chain or rope from the plug up to the surface. I think the silly ducks might have too much fun playing with that and occasionally pull it by accident, but perhaps given the right materials that wouldn't be a problem. The advantage would be faster flow and no fragile plastic piece sticking out the bottom - not a huge problem for me, but I certainly have to avoid dropping the tub spigot-down onto any hard surface.
Update, July 16, 2013:
We've got three new ducks, almost old enough for full submersion, so we're about to outgrow this setup. Therefore, coming soon: Duck Bathtub: Backyard Duck Habitat 2!
Video (duck vs. cat):