What marine animals exhibit deep hibernation/aestivation?
Fish are ectothermic, and so, by definition, cannot hibernate because they cannot actively down-regulate their body temperature or their metabolic rate. However, they can experience decreased metabolic rates associated with colder environments and/or low oxygen availability (hypoxia) and can experience dormancy. For example, epaulette sharks have been known to stay dormant for long periods without oxygen (reference) Turtles in freshwater lakes have been known to hibernate by burying themselves in mud (reference). The mechanism through which turtles can experience several months of hypoxia or anoxia is explained in this paper. The Antarctic fish (Notothenia coriiceps) has been found to be able to activate and deactivate their metabolic processes independent of body temperature and hence enter a state of dormancy similar to hibernation through metabolic suppression (reference).