Put it one a timegrapher and check amplitude, beat rate & beat error in different watch positions (watch dial up, dial down, crown up, crown down etc.). There are timegraphing apps on the iOS/Android app stores (using the microphone for detecting beat signals) for a very quick test. The watch's tick signal may be a bit on the weak side but i usually get some data in a reasonably silent environments with direkt contact of the watch to the phone's body.

https://en.wikipedia.org/wiki/Watch_timing_machine

Seiko 5s have clear back covers generally, and a water intrusion should be pretty visble, in my opinion.

Oil age maybe a factor, but maybe it’s dropped? I had a Swatch with an ETA movement (with shock absorption nonetheless), and I somehow managed to damage its balance wheel assembly by dropping to a soft carpet from ~80cm, because it started stopping when it was not in dial up position. They even opened it and recalibrated and oiled it, but it’s dead.