He kept Wang Mo in his palace, gave her something and gave her an elixir, all like protecting small animals. Although they look alike, they are not the same species. A man has lived for thousands of years, and time is slow for him. Pang Zun is right. Wang Mo is just a doll of the water prince. Just Rainbow Mo's love for Li Luo.
Besides, I think the water prince has never been hard on Wang Mo. He just thinks that human beings are creatures with limited life, so they treat the king a bit like taming animals.
The proof is that he always likes "blood skin", just as we like the soft and white fur of rabbits. He never said that he likes Wang Mo, only that he likes human girls.