So I took my yacht out for a three hour tour last night to see what's out there. It looks like the Diamond Mansion is on an island and there are two other islands in the distance. One doesn't appear to have anything of interest but I found this on the other:
It would appear there is someone living on this island. Unfortunately, I wasn't able to find any solid ground to walk on, otherwise I would have raided the fridge, erm... said hello.
Now that you mention it, I think you're right. It would make sense that the yacht club would be nearby. I'll need to visit there tomorrow to see if it matches up to my picture.