One liter Ziploc brand freezer bags are quite leakproof and tough (the kind where you squeeze the seal together manually, not the kind with the convenient slider on top). Other brands are not always so good (I haven't tested many though). The ones with the sliders leak a little bit at the corner, though not enough to make them useless.

I think it's best to not completely fill ziploc bags, but instead leave a little bit of slosh space. However, I don't think anything bad happens if you use a 1 liter purification tablet in 0.9 liters of water.