Dehydrated horse question.?

Before I start....It's late at night so the vet is closed. They are eating, urinating, seem alert etc.. so I am not overly worried 'Yet and I will call the vet in the morning'. I have two fields. The 2 horses in one field are fine. I have a mare with a not quite, 3 month old foal and a yearling in the other field. The mare seems slightly dehydrated (her foal does not) She is still making milk though. The yearling has me a little more worried. When I went to see them late this afternoon, the skin on his neck rippled and stayed that way when I petted him there. It has never done that before that I had noticed. That is when I checked the others. This led me to wonder about their water and weather or not it was too close to the electric fence. I then put out new, fresh water in the middle of the field in totally different pails and they did drink a little, but not a lot. I am not certain that it was the fence causing them not to drink but I will move it tomorrow after I scrub it and add clean water ( it was not dirty but I am going to drain it anyways just to be certain). I guess my main concern is that they didn't seem thirsty enough to gulp down the new water I put out for them... am I getting myself scared over nothing, or is it normal for them to only take smaller amounts if they have not been drinking for a while? What steps should I take to be certain they are getting the amount they need to stay hydrated? Should I give them any type of electrolyte, or something? I have never experienced this before. Thanks in advance for any suggestions. And to any that aren't positive....well just don't leave them.