Although water bottles are generally built to have breakaway caps that cannot be resealed after they've been broken, there are in fact a number of ways to reseal them. None are particularly easy, but all can be accomplished with a minimum number of tools. The trick to resealing a bottle is to not break the tamper-resistant seal in the first place, but to remove the entire cap and seal as one piece that can be put back on the bottle. Before resealing your bottle, ensure that what you are doing is fully legal.
Heat a container of water until it boils. Make sure you have a container large enough to fit the cap of your water bottle.
Dip the cap of the water bottle in the water so it is fully submerged. Leave it submerged for a minute.
Remove the bottle from the water and check to make sure it has softened.
Unscrew the cap very slowly, applying steady pressure between the bottom part of the cap and the cap itself, to keep the two pieces connected.