Is this Water Hemlock Dropwort in my garden (and what to do if so)?

My understanding is that Water Hemlock refers to the genus, so it is actually a generic category for 4 different species:
"Cicuta, commonly known as water hemlock, is a genus of four species of highly poisonous plants in the family Apiaceae."

Unfortuantely, reading more, the species I linked to (Conium maculatum) is not within that genus. Sorry for setting you on the wrong track. It is still poisonus of course, but another wiki page says explicitly that they are not the same thing:
"Similarly, the plant [Conium maculatum] should not be confused with cicuta (commonly known as water hemlock)."

Incidentally, maculatum stand for spotted, so the ones other members mentioned was probably Conium, not the Cicuta (Water Hemlock).

Cicuta is a perennial apparently, while Conium is a biennial. So they might well be a bit different in other respects too.

This is confirmed in this US guidance:
Water hemlock (Cicuta douglasii) : USDA ARS

Water hemlock may be confused with poison-hemlock because of the similarity in names; however, these two are different plants that cause different types of poisoning

The plants, which usually grow in small patches, are easy to locate. Spraying or grubbing can eradicate them. Actively growing plants can be controlled with 2,4-D applied at the rate of 1 kg per acre of acid equivalent. Repeat spray treatments until eradication is completed. Follow all precautions for handling herbicides. If you grub water hemlock, be sure to get all of the plant, including roots. Gather and burn every part.

There is some advice, but it seems to involve herbicides.

The best one might be this:

"
Mechanical control
Hand-pulling Western waterhemlock is effective when the soil is moist. Be careful to pull the entire plant, including all roots, and dispose of it in a garbage bag. Several articles recommend burning hemlock after pulling, but the smoke may contain toxins that could cause severe illness and even death. Be sure to wear appropriate gloves, as contact dermatitis is possible. If your hands come into contact with the plant — especially the root — and then touch your eyes or mouth, you can become very ill.


Chemical

For western waterhemlock, application of chemicals is most effective in late spring or early summer. Several types of herbicide can be effective, including Glyphosate, 2,4-D, and picloram and 2,4-D, and MCPA. Glyphosate is nonselective, so exercise caution to minimize injury or mortality of desirable plants that might help suppress new poison hemlock seedlings.


Keep animals away from treated plants for three weeks after spraying.


Also, after Western water hemlock is sprayed with herbicides, an increase in palatability can occur. Keep animals away from treated plants for three weeks after spraying. Chemical treatment of Western waterhemlock may require repeated applications to deplete the seed bank.


A word of caution: Herbicides sprayed within 50 feet of a body of water can pose significant environmental hazards. These situations typically require an aquatic herbicide. Be sure to contact your local weed control authority or OSU Extension office for specific herbicide recommendations. Check the herbicide label for restrictions.


Both poison hemlock and Western waterhemlock plants require caution when handling. Constant vigilance is required to eliminate these toxic weeds from your property. A good manager can contain or eliminate hemlock in order to keep your livestock safe."

Poison hemlock and Western waterhemlock: deadly plants that may be growing in your pasture


You do well being cautious with this stuff, stay safe..