My movie idea

[Giant Communist Robot]

Great white sharks mutate and sprout legs; run up on land and start eating spree. I can't wait to see this!

[OperaTenor]

You can sell it if you say the mutation was caused by climate change....

[jamiebk]

Great white sharks mutate and sprout legs; run up on land and start eating spree. I can't wait to see this!

There was one film that came close:
http://www.imdb.com/title/tt0149261/
http://en.wikipedia.org/wiki/Deep_Blue_Sea

It was called "Deep Blue Sea" and was a story where Samual L Jackson plays a businessman who sinks $200 million into a special project to help fight brain cancer. As part of this project, medical biologist Susan McAlester (Saffron Burrows) rather naughtily figures out a way to genetically embiggen shark brains, so that cancer-battling enzymes can be harvested. However, the shark subjects become super smart and decide they don't much like being cooped up in pens and being stabbed with hypodermics, so they figure a way to break out and make for the open sea.

You can guess that a lot of humans get in the way, and since the sharks now posess a brain that thinks like a humans...well...you can guess how ridiculous this gets.

[Shapley]

The Sci-Fi Channel had a movie a few years ago along similar lines. If I recall the premise, scientists travel to a remote Caribbean facility, formerly owned by the U S Navy. The find a shark/human hybrid creature that had been bred there back when the Navy ran the facility, which attacks people both on land and in the sea.

Then, of course, there was Saturday Night Live's Land Shark routine...