Ticks, yes, common in southern B.C. On Vancouver Island our dogs often transported a dozen or so home after a walk in the bush. I never hosted a tick while we lived there but had the honour of removing one from my hubby that was embedded very high on his leg, near his groin! UGH!
Swimmer's itch
http://www.healthlinkbc.ca is when Schistosomes larvae get on human skin if you are swimming or wading in water. The larvae burrow under your skin, however, they cannot survive in humans and will die almost immediately. It is the allergic reaction to these tiny larvae under the skin that causes swimmer’s itch.
Leeches, hmmm. Some lakes, yes. Even in Yukon it is possible to get blood suckers on your skin. Handy to have salt to put on them if that happens. If you try to pull them off your skin, the head might stay embedded.
I swim in lakes, drink natural water, hike in the bush ... haven't had a problem. Be aware, but not paranoid.