All life started in the sea. Transitioning back to semi-aquatic or fully aquatic has happened many times. Is it possible to transition back? According to these researches, it seems like a one way gate. The types of changes that land animals make to become fully aquatic again seem to lock a lineage into the water forever. Increased body mass and carnivorous diet are helpful going back to water but make it really hard to go back to land and redevelop mobility.