It's not what you think it is. We know that Adonalsium was split by the now shards and it was also said that there was a force that opposed Adonalsium. Sanderson considered a group of people to be a "force", implying the original shardholders. Also, the weapon that Preservation was talking about was the Atium cache which would be burned by the Atium Mistings, denying Ruin's means to end the world.
The sixteen part is also a reference to the number of shards split from Adonalsium and the number of metals in Allomancy. The number 16 is a constant in Sanderson's Cosmere and shouldn't really be thought about too hard. If there really was a weapon like that in the cosmere, it would be too much of an investiture beacon and probably would've already been found by now.
And finally, in regards to the boiling point of water, that wouldn't work either as that can change depending on atmospheric pressure, as atmospheric pressure changes depending on high up you are. Also unit changing is also a problem. Preservation realised this and decided on 16, as it was the cosmere constant--something He thought they should recognise, but at that time His mind was also pretty much gone.
If I can find the interviews which I got this information from, I'll edit this and post them below.