Loeb's primary argument that I can remember was that the object accelerated in a peculiar way as it moved away from the sun. This was thought to be attributable

Loeb's primary argument that I can remember was that the object accelerated in a peculiar way as it moved away from the sun. This was thought to be attributable to off gassing caused solar radiation. However, no gases have been observed and other analyses have suggested that this would have caused the object to spin--perhaps enough to tear it apart. Edit: I realized that you were asking about the object at the bottom of the ocean, not Oumuamua which was the subject of my response. But the two are related. If, as Loeb believes, there is evidence that the first extra-solar object that we ever closely observed, Oumuamua, was extraterrestrial technology, then the limited number of other known extra-solar objects also become of interest. We don't know what proportion of extra-solar objects are, in fact, alien technology, but Loeb's views about Oumuamua give him reason to think it might be reasonably high, high enough to warrant an expedition to the bottom of the ocean. We also know that the object is not a simple rock fragment because we can infer its density, which appears to be greater than iron.