How an underwater world could have been possible in Island Living

Yes, and most importantly, it would give mermaids a habitat.
Playing a mermaid and not seeing anything underwater is frustrating. There's nothing special to do as a mermaid that you can't do with a sim apart from like 4 interactions that have little real impact anyway.
They sleep by back floating, that is just crazy. At the beginning I found it charming but then, I realised it was the same animation as back floating but with the Zzzz sign. Mermaids should sleep under the sea. And they should have a place to call home. They should feel out of place living as normal Sims and they should be able to travel to their secret mermaid lot deep in the sea.
And then, as you say, if they like a SIM they can show them their "world".

I played with a mermaid Sim for two days and then it got so boring I decided to start a new household with just a normal human Sim since mermaids can’t even snorkel.

I bet they didn’t include it because less than 10% of players visited the other secret lots. (Fact by the Gurus)
Not that it’s an excuse. I think it’s lame mermaids don’t have a location for them. They are sims with a tail.

I'm honestly not sure what the point of those hidden lots are? Like you can fish there and dig, and collect plants but you can do that anywhere? Plus as far as I know, there isn't anything directing you to these hidden lots. Maybe if they patched in an aspiration to accompany the secret lots players might know they exist or maybe give us a reason to visit them? As far as I'm aware those lots do everything a park can but they're more annoying to get to.

Plus a dive lot I don't think would have been considered a secret lot, it would be more considered what the movie studio (GF), hospital, and police station are... Maybe the could have treated it like an active career where you can choose to follow your sim under the water? If you were visiting to find mermaids then follow, or for special collectibles... or for mermaid kelp seashells, etc, but I still think with a closed world, not including a diving lot is a huge missed opportunity.

I had hoped the underwater lot was there and worked like the Belmesia Jungle.

"MidnightAura;c-17141000" wrote:
I bet they didn’t include it because less than 10% of players visited the other secret lots. (Fact by the Gurus)
Not that it’s an excuse. I think it’s lame mermaids don’t have a location for them. They are sims with a tail.

I can understand why. There's literally nothing to do on the majority of the secret lots. Most just have collectables but if that isn't your thing then there's really nothing there but stuff to look out. Seen it once seen it all.

"MidnightAura;c-17141000" wrote:
I bet they didn’t include it because less than 10% of players visited the other secret lots. (Fact by the Gurus)
Not that it’s an excuse. I think it’s lame mermaids don’t have a location for them. They are sims with a tail.

That would explain why they stopped including them and we only got secret lots with the first EP and GP.
I think if a secret lot has more to offer than just a few collectibles, they would be more popular. A secret mermaid lot underwater would have made this occult more interesting and unique.

"Atreya33;c-17141946" wrote:

"MidnightAura;c-17141000" wrote:
I bet they didn’t include it because less than 10% of players visited the other secret lots. (Fact by the Gurus)
Not that it’s an excuse. I think it’s lame mermaids don’t have a location for them. They are sims with a tail.

That would explain why they stopped including them and we only got secret lots with the first EP and GP.
I think if a secret lot has more to offer than just a few collectibles, they would be more popular. A secret mermaid lot underwater would have made this occult more interesting and unique.

Yes, that's the problem with only looking at numbers. People don't visit the secret lots because there's nothing to do there, not because the concept itself puts them off. If visiting the secret lot in Sulani was only possible as a mermaid or as a mermaid's friend and inside was the only place to find the kelp that turns you into a mermaid for example, that would be more appealing. If it was underwater, and therefore unique in its existence it would also make people want to visit it. And if you play as a mermaid, having that lot to hang out with other mermaids and dolphins would be awesome.

I am actually one who enjoys the secret lots, in part for their mysterious lonely atmospheres and in part because they remind me of fun secret lots in earlier versions of the game (I especially liked those hobby lots in TS2). I agree with everyone else that they could have been better implemented, though, with at least one aspiration (perhaps under the Nature category) that involved finding and exploring secret lots. and maybe some unique collectibles or a profusion of collectibles only available there, or an exclusive activity (with powerful moodlets and boosts) to be found there. It would make gameplay surrounding them much more purposeful and fun.

In Island Living, I don't necessarily mind that there is not a dive lot for all sims, but I would absolutely have loved a secret, magical-looking mermaid lot underwater (even with a loading screen), where mermaids could hang out together, find unique collectibles, fill all their needs and see unique aquatic life. It would have been a wonderful motivator to play with mermaids, and, if mermaids could bring along their friends once they had achieved high enough fitness skill and friend levels, it would even have made befriending them feel more worthwhile and necessary. In general, TS4 would benefit from more purposeful and inherently motivating gameplay.