Why Hate ?
First , I would like to say that I enjoyed this movie.
Now for the science:
The tongue parasite, Cymothoa exigua
The giant isopod, Bathynomus giganteus
Both are from the order Isopoda, suborder Cymothoida - they simply belong to different family and genus. So, to say they are completely different isn't completely correct.
The smaller tongue parasite as portrayed in the fish on film is very close to accurate. All but the harmful effect to the fish. They attach to the gills an migrate to the tongue , replacing it for the duration of the fish's lifespan.
The larger giant isopod is mainly a deep sea scavenger , however it is also a carnivorous predator and has been known to adopt parasitic tendancies.
Many other species over human history have been thought to only exist as parasites or bacteria in the animal kingdom only to be discovered in an infected human patient. As a matter of fact, most of the serious outbreaks in history have come from animal-human cross contamination.
Therefore it is not a scientific stretch to think that these species either through a natural imperative or human meddling could in fact become the threat seen in this movie.
Miskatonic University - Go Pods !!!