Can Giverny be reached by boat from Paris?

Maybe, depending on how much time you have. Technically, Vernon has a marina near the town center, see this website for confirmation of that:
http://giverny.org/transpor/

But then you would have to get from Vernon to Giverny by surface transportation.
It does not give a travel time for getting to Vernon by boat, probably because the website doesn't know from where you would be starting.

You'll still have to work out the specifics of how to get from where in Paris to the Vernon marina, and how long it would take.

I don't recall anyone, reporting in here, who went by boat.
People here go by:
train/bus/taxi
rental car

If you're talking about a multi-day cruise on the Seine from paris to Vernon, there are some. I think they are 5-7 days one way and since they are small groups (or maybe even private hire) they are probably quite pricey.

Here is one: http://www.tomjonescruising.com/cruisingarea.htm

If you're talking about a day trip by boat - no way.

BekI,
A girlfriend and I went to Giverny by train and then by bus. It would have been nice to spend the night in the little village of Giverny. We were there in a rainy July. I forgot which train station we left from but it took about an hour by train. There was a bus to Giverny not far from the train station in Vernon.
What I remember the most (other than the lovely garden) was how inhospitable the female bus driver was . We were taking the last bus back to Vernon for the day so we made sure we were early. The bus was already there, it was raining but the bus driver would not let us on the bus for several minutes. It didn't make any sense as we just boarded the bus that much wetter.
This was another example of needing a chip and pin credit card. The day before we could not buy a ticket from the kiosk and the peopled ticket lines were too long so we missed the train that day and ended up going to Giverny on Bastille Day.
Good luck!