So I went back to the secret spot yesterday, and I had a solid signal which turned up an extremely crusted coin. going by the signal and the patina I knew it would be silver, but it turned out to be an extremely thick crust. I soaked it for 24 hours and still the pantina was a featureless mass of sand and silver oxide. A gentle tap and it cracked the outer shell, and out popped a british 1896 shilling.
Now considering perth was only founded in 1829 that's a seriously old coin for this area. It's likely been in that salty water for 100 years. I've left it as is, I think any attempts to clean it more would just remove detail.
I'm definately going back again with a focus on that specific area as it's where I found the spoons as well. If I can find one in good condition they can be very valuable, and I'd be stoked to find something of historical significance I can donate.