A good thesis statement clearly states the goal of your essay: generally, an essay tries to prove or disprove (or demonstrate without judgement) a central point. For an essay on tidal waves, you should find some specific point to make about the subject and then defend it in the body.

For example, one very good topic is the difference between tidal waves and tsunamis. The two terms are often used as synonyms, but mean very different things in reality: a tidal wave (or tidal bore) is the movement of water from tidal forces in narrow bays or rivers, raising the water line; a tsunami is the sudden rise and fall of surface water in the ocean, causing a wave that spreads far and can be extremely destructive. If you wanted to use that topic for your essay, your thesis statement could read: "Tidal waves, while often confused with tsunamis, are more mild and occur on a regular basis." That is a simple form; you could come up with a more scientific thesis after research.

On the other hand, you could write a thesis statement that you then go on to disprove in the body; if your subject is actually about tsunamis (often incorrectly referred to as tidal waves) you could write a paper about the destructive capabilities of both, and how they compare; this would be an interesting paper because of the common media misinterpretation of tsunami as tidal wave when there are no tidal forces involved. Your thesis statement could read: "Tidal waves from the ocean often cause great destruction." You could then go on to cite examples that show how it is actually tsunamis that cause great destruction, thus disproving your thesis while still making the central point. This is a valid direction for thesis statements, although most people try to defend rather than attack their own thesis.

Good luck!