Pea size is wrong.well I never heard of putting it in water. that sounds like it would be a waste or possibly overdose them.
I use ivermectin paste horse dewormer and give only a pea sized piece to each chicken 1 time. and have never had to treat them 2 times.
this is what i use
https://www.tractorsupply.com/tsc/product/durvet-ivermectin-paste-187-608-g