congestive heart failure maybe?

http://www.meyerlandanimalclinic.com/topic010.htm

On Monday I took my FeLV+ 5 year-old male longhair to
the vet, with these symptoms:

Extreme sudden weight loss.

clear fluid discharge from the eyes and nose.

Fluid build up in the body cavity.

Lethargic, gradually increasing over period of 1 month.

After a cursory 5-minute exam, the vet stuck a syringe in
his abdomen, and drew out a clear, yellowish-tinted fluid.

Immediately, he concluded that it was a cardiological condition
that was terminal, and that there was no hope for the cat. He
said I should put the cat down, and that the only thing he can
do for it, is to drain the fluid from the abdomen, to make the
cat more comfortable, and I asked him to do that. He drained
1.5 pints of fluid from the cat. The cat is on antibiotics and
another pill which may be for pain.

(continued)...

This sounds a lot like FIP to me...the fluid buildup is characteristic
of the final stages of effusive (wet) FIP.
FIP is terminal. IIRC, it's an autoimmune disorder where the body begins
attacking its own tissues. There is no conclusive test for FIP. Only a
biopsy (which means surgery) can tell for sure.

I'd always suggest a second opinion. Vets are not gods - they can be
wrong, they do make mistakes, and sometimes they're overly opinionated.
IOW, they're human.  :)

I hope Rocky pulls through...