You may be able to reduce the obviousness of the texture stretching by using a texture with less contrast in it.
For your invisible wall issue, could you create a script that kills/harms players that stray too far from origin of the map if they are in a mini-game?
Also, the beach doesn't have to extend all the way around the island. It could be more interesting to have a few seaside cliffs.
On the subject of beaches, I think you may have your textures reversed. Typically the sand closest to the water will be the finest and the coarsest will be set back from the beach.