How the heck do you call over a dolphin?

Click on the byoy, I think. I tried aiming at the dolphin and at the school of fish, but I think what worked in the end was the byoy itself. You should see the option "Call dolphin". I had to try twice, after the second time one came up. It didn't stay for long, I called it a third time, it came up again and then we got... company.

I think there is only a small window like twice a day they appear by a buoy. There will be a swirly thing under the water under the buoy with dolphins and fish swimming around click on that. It was say call dolphin. I had one come up out of the water to try and talk to me in it’s own so that another thing. Hope this helps and have fun.

You can only summon a dolphin if your Sim is a mermaid or the lot has Oceanic Paradise lot trait. Otherwise they only spawn at the buoys randomly.

I did figure it out. It requires multiple clicking right near the buoy. I think the school of fish being under it may be an indication that you can have the interaction. I also had a shark suddenly show up once and scare the dolphin away.