I have a Cocker Spaniel that is acting extremely out of normal character, help?

Your dog is elderly (this does NOT mean his lifespan is anywhere near the end, he could live to be 20), so his back is more fragile than it used to be. He could have slipped a disc, or could have arthritis, or even muscle pain. Your vet will be able to figure that out.

Frequent drinking (more than is usual for him) can be a sign of diabetes, which is common in older animals (dogs and cats both). It can be easily diagnosed by your veterinarian, and the treatment can be anywhere from scheduled measured meals to keep his glucose stable and not too high or too low, a pill, or an insulin jab (pets get used to all three of these treatments easily, and it can be very cheap). He may just be getting dehydrated quicker due to having less energy. His eyes being tired or wet could be due to less energy, or needing more sleep, or maybe he's deficient in a specific vitamin.

Make sure you tell your veterinarian about all of these things, maybe make a note of it all to bring with you: His back, and where it hurt, and how he acted afterwards; His increased drinking habits (and your concerns about whether or not it could be diabetes, though they will likely check already), and his "drowsy" sleepy/tired eyes and them looking wetter than usual.

Ah the poor dog. Could be a number of things, as Daïtro pointed out, slipped disc, arthritis, muscle pain. Can he jump? Just watch him because cocker spaniel can get this thing called Intervertebral Disc Disease. Just do a bit of research on that. The water could be due to diabetes, is he eating? I really hope he recovers soon. Best wishes to you and your dog.

No one, including a vet, can diagnose/treat your dog without examining it, including YA.

A dog of this age needs a comprehensive wellness exam if one hasn't been done in the last six months.

The exam should include a complete blood work up, urine/fecal, dental and if indicated, UTD vaccines, x-rays, and an echo cardiogram.

No, we can't, and even if we thought we could, what good would it do? We could tell you something it could be and then when you get to the vet you find out it's actually something entirely different.

Probably hates living with you