Diving and hypotension

Is low blood pressure a contraindication to scuba diving?

Generally low blood pressure is much less of a problem than high pressure, in relation to diving.
The reason is that, during water immersion, the blood pressure levels are more equilibrated than at surface and the risk of orthostatic hypotension (the fall in blood pressure experienced while standing up from a sitting or recline position, with the related dizziness, possible obnubilation or even loss of consciousness) is almost non existent, due to the fact that the effect of gravity is countrerbalanced by a blood re-distribution in the body and the dimished effect of the weight of the blood liquid column. The blood circulation becomes "centralized" and hydraulically similar to the situation found in the supine position. This assures that the brain is not easily subject to sudden falls in blood perfusion and oxygenation.
Generally speaking, if your low blood pressure does not impair your normal performance on land, there should be nothing to worry about while in water or underwater.
I would advice you to undergo a thorough clinical and fitness to dive check, including a cardiac and pressure performance test with a cardiological evaluation under stress.
Furthermore, as the blood pressure is also related to optimal hydration and blood salt contents, I would highly recommend that you keep yourself constantly and adequately hydrated and that you pay attention to the assumption of adequate quantities of salt and electrolytes, especially during the hot season, when fluid and salt loss through perspiration may be very significant. Please refer to any local physician or drugstore to find the available products.