I travelled to Kenya last summer for 6 weeks and got Malaria, and I was on anti-malaria pills!
Anyway, I'm not a dr. but I would say don't worry about the pills. You are only going to be there for a day, and generally, it takes longer than that to get Malaria. Yes, you can get it from one bite, but my other thought is, if there had been any outbreak of Malaria, they would either cancel that excursion, or inform people of the risk.
Yeah, Malaria isn't fun, but I really think your risk is very low.