Parasitic leeches would be attached to their hosts, not driving around the pond. They are most likely detritus-eating leeches that are eating dead and dying plant and animal material. As far as parasitic leeches go, those that attack fish are pretty common but, since you have no fish, they probably aren't in the pond. That species does not suck on humans. Most human-sucking leeches are tropical species and not found in the US. Where do you live?
What do the plants look like? Some plants would be good. Others can be invasive. Various animals like the ducks, some fish, turtles, etc. may eat some plants. It would help to identify the species of plants in the swimming pond to determine if they're good to have or if they should be pulled up before they get out of hand. I advise avoiding herbicides if at all possible. They do a lot of harm to amphibians and other animals including us. The ducks will add fecal bacteria to the pond but, unless you have an immunodeficiency, your body will easily fight off such bacteria. Most animals bathe and drink from ponds that are full of various animal wastes, dead and live animals and plants and such, and they do fine. Since your pond is spring fed, it will get constant water changes to dilute any pollutants.
We'd love to see your swimming pond. My new site,
http://www.pondshowcase.com lets you put up photos and information about your pond. I even have a swimming pond category (with no entries).