The phrase "bobs up" typically means to appear suddenly or unexpectedly. As for antonyms, there are several options, depending on the context. In certain situations, the opposite of "bobs up" might be "disappears" or "vanishes." In other cases, it could be "descends" or "sinks," implying that the object or person in question is moving downwards instead of upwards. Alternatively, one might use a phrase like "remains hidden" or "stays below the surface" to convey the opposite of "bobs up." Overall, the antonym for "bobs up" will depend on the intended meaning and the specific words and phrases used in context.