I chanced upon two Chinese characters that signify both water with profit, but how can they possibly relate to each other semantically? Have the Chinese traditionally privatized or invested in water? If so, average lay Americans aren't that apprised of water privatization or investment.
We have to use some linear thinking to see the extended meanings of many Chinese words.
wet → moist → sleek (things that get wet become moist, a moist surface become sleek)
to make sleek → to polish → touch up (one way to make something sleek is to polish it. A polished item is touched up)
wet → profit (imagine a plant that's drying up. When you water it, it absorbs the water and it is enriched. Which means 'profited'. And that is how 潤 acquired the meaning of 'profit'
Now to answer your question: "how can they possibly relate to each other semantically?"
They acquired the same meaning independently, like a convergent evolution of birds and bats developed wings independently.
I have explained how the character 潤 acquired the meaning of 'profit'.
The character 益 (overflowing) also acquired the meaning of 'profit' by extending its basic meaning. The character 益 depicts water overflow from a vessel. It means there is excess water. And 'excess' means 'more than required'.
Imagine you put a certain amount of money into a business, it requires an equal amount of money in return for the business to break even. To make a profit, the money in return must exceed the amount that is needed for it to break even. That is how 益 acquired the meaning of 'profit'.
