That's a nice question. The two are pretty similar, in that they describe an unoccupied location, but they are not quite the same.
When we say a place is "secluded," we mean that that place is not often occupied because it is "out of the way," or hard to get to, or sometimes just not well-known. I don't think peace or danger has anything to do with it. For example, when you hear about crime stories, you often hear that the murderer hid the evidence in a secluded location. But from a tourism perspective, where many tourists don't want to be surrounded by other tourists, finding a secluded spot is a positive.
When we say that a place is "deserted," we literally mean that the place used to be occupied, but that it no longer is occupied. A synonym for this literal meaning is "abandoned." However, "deserted" can sometimes be used metaphorically to mean that there aren't very many people somewhere. In this case it is a synonym for "unoccupied" and need not be negative. From a tourism perspective, it could be negative or positive, but is usually negative. For example, "we were excited to go to the state fair, but when we got there, it was deserted"-- this suggests there was nobody there and it was no fun.
A tourist agency may emphasize that a beach is secluded to tell you that it will be a nice place to go. In this case, "secluded" will be used more often than deserted, because deserted implies that people are leaving the beach for some reason, and the tourist agency might not want you to ask why! But again "deserted" doesn't have to be negative here, and I can see a tourist agency using either to describe a beach.