This is one of a series of questions centered around how an isolated group of people would survive. Each question focuses on a single aspect of survival. Details about the peoples' situation are below:
In a novel I am developing, a village's worth of people is living on a peninsula. The isthmus connecting the peninsula to the mainland is very narrow, and spanned by a wall, which prevents the people from leaving (there are deterrents preventing them from climbing the wall or otherwise circumventing it). They also cannot swim around the wall. This also means that no land-based animals can cross onto the peninsual from the mainland. The inhabitants have to live with what they have. For the sake of details, assume the peninusla is roughly the size, shape, and location of Mahia Peninsula.
This particular question deals with water being an escape route. For the moment, assume that there is no way for the inhabitants of the peninsula to approach or get across/around/under the wall. The wall is not an option. Flight has not yet been dreamed of (think 13th century). The only remaining option is the sea.
It is very important that the inhabitants have no way off the peninsula. This is vital to the novel. The sea cannot be an option for escape, and there can be no doubt about this. What can I do to make sure escape via water is completely out of the question?
For technology purposes, assume the inhabitants of the village are peasants of the medieval era, perfectly adept at building.
I've marked the answer by Lu22 because I feels it provides the most air-tight deterrent. Everything else, while great, still has the problem that given enough attempts, someone is bound to succeed in the end (granted no one will know that he succeeded, but still).
I would like to mention that the answer here has a convenient index of sorts of everything listed. The answer provided by Strongo was also a very helpful one, and combined with sea monsters I think would be an awesome combination.