The presence of Yukikaze in the manga. The precedent so far is that the Fleet of Fog will need the sunken hulk of a particular ship as either templates of reconstruction (the whole body is nanomaterial) or 'corpses' to reanimate (the body is part original, with nanomaterials repairing/patching existing damage, altering the 'chassis' for incorporating new features, and making it work). Yukikaze was given to the Republic of China after WW2, renamed into Tang Yan, and served until the late sixties before being scrapped in 1970. This means the current Fleet of Fog Yukikaze should not have existed as there was nothing left but schematics (if any). Of course, the series might just handwave this as Alternate History at work.
Hyuuga was also broken for scrap in 1946, although she does spend almost a year as a derelict ship (she ran aground on shallow water) thus not fully out of precedent.
Correlation does not equal causation. The very presence of Yukikaze and Hyuga disproves the reanimated sunken hulk hypothesis, which has never been explicitly stated in the manga. The anime, at any rate, does not appear to follow this hypothesis, given the presence of twenty nineNagara-class light cruisers (only six were built in reality) and a Fleet of Fog Iowa-class fast battleship (all four Iowa-class battleships were never sunk and are serving as museum ships).
New Jersey is now also confirmed as a Fog Ship. The "reanimated sunken hull" theory is dead.