It is generally thought that there are three forms of oceanic phobias: fear of deep water, fear of open water and fear of seawater. Fear of deep water is the most generalized form, being the basis from where all other fears derive. This type of fear is not a very specific one, as it is shared by many animals with common ancestry; such as terrestrial vertebrates and mammals in general. The same can be said for fear of open water. However, the third form of thalassophobia (fear of seawater) seems to exist only in humans who have been conditioned to associate danger with contact or ingestion.
It manifests as a quasi-genetic predisposition to avoid bodies or depths or any such featureless space without immediate access to firm ground (be it concrete or rock) to which one can cling should something bad happen.
Since the human imagination is a powerful tool, it is not surprising that this type of phobia manifests as more than just an initial reaction to contact with water.
Rather, fear of seawater becomes an obsession that drives humans to underestimate their own potential and overestimate the danger of anything aquatic. This type of phobia is not limited to only people, but also manifests in other terrestrial mammals. The method by which this fear is transmitted through the generations might be explained by epigenetics and learning.
Epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence. Such mechanisms include chemical modification of DNA, histone post-translational modifications (DNA methylation), and miRNA-induced silencing. Such epigenetic processes can be influenced by environmental factors including parental care, toxins, nutrition and exposure to pathogens.
Although epigenetic modifications can have both positive and negative effects, the fact that they are heritable makes them useful for explaining phenomena such as transgenerational trauma. Giving birth is a very dangerous process in mammals, but to pass on knowledge about predators from one generation to another it has been evolutionarily advantageous for parents to not be too concerned about their own safety. Thus, natural selection has resulted in many species of mammals being born with an innate fear of water.