Well, vertigo is actually a symptom, not a condition in itself. Therefore, there are a number of triggers that provoke the reaction. If you rupture the oval or round window in the inner ear due to barotrauma injury, vertigo is pretty much a given, and it's true vertigo that may persist even out of the water. For transient vertigo in the water (alternobaric vertigo, which is relatively common), it's usually a middle ear thing, with unequal pressurization of the two ears (which may be caused by wax buildup, deviated septum problems, eustachian tube disfunction, congestion, etc.). Sensory deprivation can cause feelings of vertigo, and in diving this can happen in murky or dark water, for example. Stress, such as swimming against a current and breathing hard, can produce high levels of carbon dioxide, which might give a diver a light-headed feeling along with vertigo. Nitrogen narcosis can result in vertigo. OTC drugs might have narcotic effects at depths that cause a feeling of vertigo. Vertigo underwater is always risky, so self-rescue in terms of recognizing the feeling and having some strategies for dealing with it are important.