It really has nothing to do with increasing water in the joints. You may see some increase in synovial fluid, but if there was a lot you'd actually get pain from that. You are simply better able to repair the minor damage that occurs to these tissues during regular lifiting while taking the Deca. The pain once you come off is almost certainly related to the shift in hormone balance caused as you come off your cycle and enter PCT. If I knew your cycle and PCT protocol I'd be able to tell you exactly why you get the pain and might even be able to advise a change or two that would prevent it.