I understand the layer feed at this age as this is what most feed stores reccomend, if they are done with medicated, then they go on layer. Realizing the difference now, this can be changed for all of us.
The next thing, water problems, can you go out with a spade and make a small trench to direct the flow of water from your chicken run/house. It will be the best idea all around to do this for your own property usage and hens. I too have a water issue from natural springs and have to create drainage of shoveling a 6" deep x 4" wide, then fill it with rocks or sand if available, cover with dirt to grow grass. Or cover with a board. Rocks are better as if it goes stagnant, you don't want to have molds and misquetos growing in it.
Sadly, your birds that have died, who knows, it could be a million things that you are not aware of- nor am I! Chickens do not like wet feet, if the ditch isn't enough to drain the water away from your chicken yard (and it could be a fertilizer or chemical in the water) also, some people have the water softeners and from my own experience, it is run through some type of sodium and it was 3 days of hell getting over those 5 glasses of water! I don't know how they are for your chickens. (Just a suggestion)
Your girls also could have picked through things, eating stones that are wet and moldy from the ground, who knows but even a small change will help. Keep your chicken(s) in for a few days, get to working on a trench and let that pen dry out, I would even take a mild bleach in a pump action sprayer, soak the ground down, let it be. Bleach straight will harm, but 1/4 cup to a gallon of water is a strong solution that will not harm anything.
You may eventually want to add sand to the mix so they can dust bath, drain water quickly.
I have no idea why your babies died, but clean up the area and see if it helps. Do not feed them anything in a open pen, it could also be wild birds too. feed only inside, fresh water inside, as they can be drinking from the ground which is not quality water - thanks to the neighbor.
Good luck to you!