At a quick glance (or until you lean the difference) Water Hemlock looks an awful lot like Queen Anne's Lace ---on speed. (its thinner and somewhat strung out.) First time I saw it, I thought, that's weird, that Queen Anne's Lace grows odd in the shade. (It looks too, a lot like Poison Hemlock and Wild Parsley, but the latter are somewhat less common than QAL.)
As is, Water Hemlock (along with Poison Hemlock) are highly poisonous, so not a good idea to mistake for wild carrot (QAL), wild parsley, or wild parsnip.
QAL and Water Hemlock often grow in close proximity and in patches, with the the difference that Water Hemlock seems to prefer the shade and ditches. Along highway 64, from Conway To Mayflower, QAL seems to dominate the open eastern side of the street, while the Water Hemlock sticks to the the western, darker, ditchy, heavily shaded side. I've seen a few Water-Hemlock plants in open pasture, but they seem to be the exception. Beyond that it seems that the Water Hemlock are with us for a shorter period of time, though this year its a long- short time, given the rains.
Beyond that, Water Hemlock and QAL share certain structure elements. Both feature bloom clusters within clusters - called "umbels." (Bloom clusters radiating on stems) Think of fireworks, or galaxies ... breaking forth atop of a larger Firework boom. (One difference however, QAL appear to unfold into a disk, while the WH umbel pushes out, from a big-bang disk to a bulbous igloo.
WH are just a blast to photograph. In as much as the umbels are very dimensional (and often at various stages of growth), I have pushed the focus vector through a single plant and watched as various umble parts explode as one part sharpens then fades, reaching from cluster to cluster.