Most organisms decay in water. I soaked goathead seeds in a covered container of water for a full year. The water developed algae and became green, but at the end of the year, the seeds were as strong as ever. They did not decay at all.
I have collected every seed I could find that I have stopped on, so I had plenty to experiment with. I used readily available household cleaners including dish detergent, vinegar and salt water.
It looks like the seeds can tolerate salt water, for a short while anyway.
The most impressive results came from the seeds soaked in straight bleach.
The picture does not do justice to the beauty of the decay.
Few seeds are easy to drop into a jar of bleach, but the same result could likely be achieved by soaking a blanket or towel in bleach and covering it with plastic to keep it from drying out so fast from evaporation.