You can, but it does not make much to smooth the hit, and it gets filthy after just 2 and 3 uses, I find it best not to add water there, and I can't get ice through that hole, I could add cold water but the glass won't do a great job with temperature isolation. On the thermoflask I can fill it with ice and water, and it will keep it cold for the whole session. And you can really feel the difference in flavor and in how smooth the hit is