P58 water bottles
P44 water bottle
Two water bottles are required for your kit.
Pattern 58 water bottles are still the standard, though now commonly referred to as Osprey which is the maker not the type. These are all plastic construction including the cup. You could consider upgrading the plastic cup that comes with these to a metal cup (out of the Crusader system, 2004) so that you can heat water or cook directly in the cup or opt for a Pattern 44 metal water bottle and cup. The Crusader mug with P58 water bottle does not fit into the P58 water bottle pouch.
For P58 webbing the acquisition of plastic US canteens were very popular as they fit in the water bottle carrier better and came with a metal cup. So these wouldn't necessarily be out of place either.
The P44 bottles and carriers were very popular since they were metal and could be exposed to heat unlike the P58 which are plastic. Additionally, the carrier was more spacious.
When purchasing your 58 water bottles be sure you get the mil issue ones; avoid ones that are advertised as British “type” water bottles. These are Chinese made and easily split at the seams.
Make sure you have a cup to go with whichever canteen you use.