Well I have just harvested some watercress that I grew from a packet of seed, in soil, and I was very impressed. It tastes just the same as what you buy, but didnt grow so big. Lovely. I don't know about bugs. That is probably from the sort grown in water, but I am sure someone else will know.

I've never grown it on soil, only in the pond and have used it for watercress sauce. You definitely shouldn't use the wild watercress that grows in watercourses where livestock drink from and poop in, as it is clearly contaminated. Maybe that's what you've heard.

You can also grow American Landcress which is very similar in flavour to watercress, though somewhat sharper. No water needed (well other than watering of course :-))

Seeds are readily available in most garden centres.

As far as I know, watercress from the wild can carry a liver fluke or some such bug that makes it unsafe to gather and eat

Commercial watercress is grown in certified-pure water,
and therefore free from liver-fluke.