To install just extract the entire package to your vice city folder. Asks for replace click on yes.
If done all things as instructed in this read me you can see and visit the underwater mini island in vice city. There are two watch towers, a mansion, a submarine along with many trees and flower plants on the island. The main thing is it is inside the water. Follow the map provided to find the island.
But when you go there you can see a mansion in the up of the entrance road. See the screen shot provided to know the mansion so, that you will not be confused. But don`t worry it`s not solid and you can go through it. Then you can see water on that road, here you need to be a little careful. Go through the light blue colored water not through the deep blue water otherwise you will die. Then you will reach the island. Hopes you will enjoy it.
Made by Gtafan2u.