I dont recommend putting wormers in water. You dont know if each bird will drink the treated water or drink enough of it to be effective. Sick wormy birds wont drink at all.
There's nothing saying that you have to worm all your birds at once.
You can worm half of them one day, the other half the next day.
OR, you can worm them by breed, for example; RIR's, BR's, Orpingtons the first day.
EE's, Sex Links and mixes the second day. Do the same when you reworm them in 10 days.
OR you can worm them by coop(s.)
It's best to go out to the coop early in the morning when it's still dark. The birds will be easier to snatch off the roost in order to administer the wormer. Also, they will have empty crops and stomachs. The birds will be hungry and so will the worms.
The worms will be at their weakest and the wormer will be very effective eliminating them.
Bring a flashlight with you. Preload your syringe. Grab a hen off the roost and cradle her in your forearm. With your hand, use your fingers and thumb to pull down on her wattles and her mouth will open. If she shakes her head, hang on and she will tire out.
With your free hand, grab the preloaded syringe and quickly squirt the liquid in her mouth and immediately let go of her wattles so she can swallow the liquid. If you dont immediately let go of the wattles, she could aspirate and you dont want that to happen.
Use no more than 1/2ml of the liquid wormer at a time in the syringe when administering the wormer. Too much liquid and she could aspirate.
Then release her and grab the next chicken off the roost and repeat until they are all wormed.
Practice doing this procedure without actually worming them, a dry run so to speak until you get used to it. Then when you're ready, you'll be confident about it and it'll go easier for you and wont take too much time at all to worm all your birds. Practice makes perfect.
The bottom line is that you'll know each bird got properly wormed, no questions about it.
You could even have someone hold the bird for you if they're willing to get up early with you.
Dont forget to reworm them in 10 days, same procedure.