Transparent objects showing through other objects

Why does my water or plants show through other objects, like other plants or my hair, and is there a way to fix this?

It's called the alpha sorting bug, though it's not really a bug. It's a limitation in the way SL renders semi-transparent objects and there isn't really anything you can do other than not using transparent textures when you don't have to. If you didn't build the item you don't really have control over this other than choosing different objects, placing them so they don't overlap, etc.

It doesn't necessarily have to be transparent textures or textures with transparent parts. ANY texture that has an alpha channel will have this problem.