Similarly, often a land parcel will have a boundary which is a river. In that case, usually the boundary goes down the middle of the river or creek and the amount of land that is under water will depend on how wide the river or creek is.
Because of issues like these, it is important to get a survey when you are purchasing land, especially land in the country. With this, items in the legal description of record will be drawn--so you can actually see where fences, bodies of water, etc., lie with relation to the land you are purchasing. A survey is a negotiable item when making an offer on land--the buyer can either ask the seller to pay for it, the buyer can pay for it himself (if it's an issue that is important to him) or the cost can be split 50-50 between buyer and seller.
Hope this helps.