Another random thought, conceived in you-don't-want-to-know-where as I watched something plop into the water:

What kind of shape should an object have to make the most minimal of splashes as it falls towards a liquid surface? A streamlined one would seem to suffice, but somehow I think it's the initial point of contact that makes the most difference. I'd go with a pointed stick dropped vertically, but weight distribution does present a problem.

It's probably been solved, but I really can't be bothered to search for the answer right now, so I'm leaving it as it is.