Blood, like other liquids, is virutally "non-compressible" at the depths we are talking about here. Thus, fluid dynamics (blood flow, urinary flow, lymph flow, cerebrospinal fluid flow) proceed at depth the same as they do on the surface. This is important also for those diving with cerebrospinal fluid shunts for hydrocephalus or with implanted liquid pumps such as those that dispense chemotherapy, narcotics, etc. In general, at recreational depths, these device function normally.
The major effect of body immersion on bp is the effect of increased venous pressure, not arterial pressure. Increased abdominal and thoracic pressure tends to affect thin-walled structures like the vena cava. The body "interprets" this increased venous pressure as if we are very fluid overloaded, thereby reducing the output of antidiuretic hormone. This drastically increases urine output as the body mistakenly tries to reduce venous pressure by dumping blood volume. This is why we often have to pee shortly after getting into the water, even at swimming pool depths. Cold also increases urinary output.
Thus, if we are already fluid depleted, or on medications that vasodilate or limit heart rate increases (like certain blood pressure meds) and then dive for extended periods, we may dump enough fluid in urine to make us hypotensive when we get to the surface.
