Debris From Space
One of the things you really don’t expect to see when you’re out on a walk with your dog is remnants of something that clearly fell from the sky. The OP described it as a 3-meter diameter disk with a top that looks like polyester in a honeycomb shape which is fiberglassed to a flimsy aluminum disk and it was found in the shallow waters of Guam.
This one took a little bit of research but they did get an answer finally. Apparently, there was a Chinese Long March 3B rocket launch that failed and the rocket was seen re-entering the atmosphere near Guam. There you go, mystery solved.