You will have frequesnt access to water on the route to lower boy scout, since you will be travelling next to a stream most of the way. From lower boy scout lake (LBSL) to upper boy scout, you will see a few smaller streams along the way. From upper boy scout (usually people turn left prior to the lake and don't actually go to it) to Iceberg lake, you will not have water until you get to the "waterfalls" (usually a pretty thin trickle this time of year) just below Iceberg lake. From iceberg to the summit, you will not have access to water. I find that most people can usually make it from the portal to LBSL with 2 - 3 liters, then from LBSL to iceberg with another 2 - 3 L and then from Iceberg to the summit with another 2 -3 L. That means that you can generally just fill up at each of the 3 major lakes along the way. There is a running debate about whether you need to even filter the water at these lakes, so make your own choice.
