What do each of these symbols represent in "The Open Boat": the sea, the gulls, and the tower?

Well, first of all, remember that this story was based on actual events and Crane a Naturalist who sought to evoke the world as it was. Therefore the details you mention need to function first and foremost on the literal level: the sea is first the sea (and only second anything symbolic), etc. Next, remember that the point of view shifts, and so these objects might mean different things to different people. To the news correspondent, for example, "This tower was a giant, standing with its back to the plight of the ants. It represented ina degree, to the correspondent, the serenity of nature amid the struggles of the individual—nature in the wind, and nature in the vision of men." That's a clear statement of meaning, but it isn't the only meaning. The others on shore seem not to care about the tower, almost not to see it. For them it means less.
As for the gulls, we're told they are "uncanny," and so they represent the weird and unpredictable: fate that could go any way. The sea is related; it is the power that determines the fate. It is the great unknown of nature.