There are no issues with the construction of the Watercell for hot water use (the seams are RF welded and the TPU film has a melting point above that of boiling water),
Use as a 'hot water bottle'
There might be safety concerns about filling a soft water container (with a threaded cap/spigot) with very hot water. The cap and the thread of the fill port are made of different materials, and therefore may expand at different rates. This could cause water to seep out - if the Watercell is inside a sleeping bag at that point, this could result in scalding injury.
For this reason, we would not recommend using a Watercell for this application.
Use of the Watercell X as a shower
If you are using the showerhead attachment with your Watercell, we would recommend a maximum water temperature of 60°C / 140°F (which is significantly above that needed for showering). If you will be adding hot water to a Watercell, we would always suggest partially filling the Watercell with cold water first.
If you are using hot water to wash out a Watercell, we would recommend water a maximum temperature of 60°C / 140°F (still well above that needed for dishwashing) - we have a separate FAQ answer on cleaning Watercells https://support.seatosummitusa.com/hc/en-us/articles/4411558493588-How-do-I-clean-my-Watercell-Pack-Tap-
Freezing a Watercell for use in a cooler
You could freeze water in a Watercell X providing you leave plenty of space for expansion as the water turns to ice. The welds, laminates and cap would not be damaged by temperatures cold enough to freeze water. (As a consideration, there are simpler and less-expensive water containers that would do the job)
BTW - in our home country of Australia, a 'cooler' is known as an 'Esky'. And across the water in New Zealand, it's called a 'Chilly Bin', which is quite simply poetic.