Mermaid play appears to be shallow overall (pun intended). Beyond what you mentioned, if you have seasons you can influence the weather and do a few other interactions like talk with fish in bowls or tanks and such. Its less in comparison to vampires in part because there is no building of powers or customizing strength and weakness which is what I think people were desiring. TBH, its not far off from aliens in terms of play with a unique CAS and a few novel abilities.
You can do a few things to make things more interesting. I created a private club where being a mermaid is a requirement and activities including mermaid-like stuff like swimming, singing, diving and such. Its fun to start a club gathering then as you will get a group of mermaids to hang out and frolic with.