Be careful putting hot water in disposible water bottles. Most disposible water bottles are made of polyester, which is PET or PETE (or polyethylene taraphthalate) and has the #1 recycle number on the bottom. While PET is a semi-crystalling resin with a melt temp well above the boiling point of water, bottle resin is specially formulated to remain amorphose, and the amorphose resin has a glass transition temperature around 150-F. Which is to say it starts to "melt" at 150-F.
I believe the Nalgene bottles are polyethylene and polypropylene, both of which are semi-crystaline polymers with melt temps above the boiling point of water. However, even though they have melting points above the boiling point of water, they also contain an amorphose phase with a glass transition temp below the boiling point of water. It's the amorphose phase of the resin that gets soft when you pour boiling water in it, so while the bottle retains it shape because the crystaline phase doesn't melt, the bottle gets soft because the amorphose phase is well above its glass transition temperature. There are further complications with polymer chemistry and melt temperatures, because different types of additives and neucleators and crosslinking, etc.. can change the softing points of these resins, and I'm sure Nalgene has special formulations for their resins. I don't know that much about polyethylene, but I'd bet the clear polyethylene bottles are clear because they're amorphose, with no crystaline phase, and hence have a glass transition temperture well below the melt temperature of PE.
Fun with plastics....