More fully, this concept is often stated as: “If you’re walking along the beach and find a watch, you just know that such a complex object must have had a creator”.
This is yet another one of those “gotcha”-type statements that creationists hope will instantly shut down any discussion about creationism and evolution.
The idea being that everything else on the beach, e.g. the water, the sand, rocks, plants, etc. are “simple” things that might have evolved, however the watch is so obviously a complex object (with a purpose and sharp metal components) that it must have had a sudden point of creation, and with a purpose.
The religious are of course extrapolating some sort of point (that they think they’ve made) to arrive at the conclusion that if an alien was to arrive on Earth and, while walking along the beach encounter a human being, then they would be (somehow) forced to admit that, due to the analogous complexity of a human being, the creature they encountered must have been created, and for a purpose.
That’s the incorrect part of the argument. And it’s also a strawman argument because no one is saying that a watch wasn’t created.
Exactly what this argument is supposed to achieve can only be guessed at because no one is suggesting that watches fell fully-formed from a tree or that they bred from a slightly different example of a previous watch. Of course they were created and therefore had a creator.
Humans evolved from primitive life forms over billions of years and therefore don’t need a creator to construct our final form. For more details of that process, see the table at A dog can only give birth to a dog (a page on this site).
Whether those primitive life forms need a creator is a different question, but it should be obvious that it is not correct to consider a human found on the beach as evidence for a creator. Complexity is the result of evolution (when it comes to humans) and human ingenuity (when it comes to watches).
At best, an alien should be led to the “must have been a creator” question if they found a protocell (e.g. just containing self-replicating RNA molecules) on a beach. Anything else that they find on the beach is explainable based on well-studied and understood processes.
The argument is also one of incredulity because the religious proponent is saying that because they can’t conceive how the complexity of a human came about, it must have happened via a creator. The incredulity comes about due to a belief in the concept of Irreducible complexity. For more detail about why that isn’t correct, see the page at the link given (a page on this site).
The “gotcha” conclusion believed to be contained in a statement like “a watch must have had a creator” comes about via indoctrination and a lack of education. Both are a pity (and are unnecessary) in this day and age.