This is a silly joke question, and you’re right about the urea breaking down. Now for the water molecules (which are indistinguishable, so outside of pseudoscience the answer is meaningless), it becomes more interesting.

Assuming a person pees about 1l/day, over a lifetime you have on the order of 10^29 water molecules in your pee. There’s about 1.3 billion cubic kilometers of water on earth, or about 10^21 litres. So assuming the pee gets perfectly diluted, you get about 10^8 molecules that used to be in a specific person’s pee, per litre of water. In reality it will likely be more, since much water is deep in the ocean, where there is little convection, so it takes long to evenly mix the pee around.

There’s alloys with lower melting points, here’s one that’s 281K/8°C

https://www.mdpi.com/2075-4701/13/3/615

Relativistic mass is not helpful to our everyday understanding of mass, it’s more helpful to discuss momentum, like the other commenter pointed out