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'.