Creative game edge-of-map ideas

I'm making a game (duh) and I need an idea for something that prevents the character from getting out of the map, but I don't want just an invisible wall. The game is set on an island, so i cold just make the ocean really large, but that seems like it would look bad. Should I have a monster of some sort or what?

 

You could just show a subtitle like "I'm the developer of this game and I don't think it's wise to continue this way. Turn back!".

And afterwards, in case player continues on everything gets pixelated and retro until it hangs.

 

Rule number one of level design, never place invisible walls. There must allways be a reason for the player to not go through certain areas....buuuuut, considering the game takes place in an island, you can place an invisible wall (or a message to return back), say, 5-10m inside the water. A lots of games do this, specially with seashores or beaches, if the game has no transport over (or down) the water. The first time the player tries to get there, he will catch the idea.

If you are thinking about enemies, maybe sharks would do the trick.

 

That's not too much creative what if Jaws theme starts playing from 50m in the sea...