i'll second cbcbd's idea of lion's head on the way down that way you get to climb out of the bowl plus see it from above. i love hiking right up tux. is this midweek or weekend? weekend that trail can be rather busy. mid-week, i have sat at the top of the headwall for half an hour without seeing a soul.
2-3 liters should be fine depending upon your level of fitness. i've done it with two but three is better if you don't mind the weight. you can refill at the summit. people getting out of their cars and off the train may look at you funny and ask stupid questions like "did you really hike up?" just ignore them
best part about climbing washington is the plateau above tux under the summit cone, don't rush that part of the hike as the summit is always a disappointment, much better views from just above tux, imo.