Final answer:
With proper water temperature and hydrostatic pressure, the heart rate is likely to decrease due to assisted venous return and avoidance of extreme temperatures that could drastically alter the heart rate.
Step-by-step explanation:
When considering the effects of proper water temperature and proper hydrostatic pressure on heart rate, a few key physiological responses occur. In the context of aquatics and heart rate, both factors can lead to cardiovascular responses that are generally aimed at maintaining homeostasis. The hydrostatic pressure of water, especially when submerged, applies a pressure on the body that can assist the venous return to the heart, thus potentially affecting the heart rate.
When a healthy young adult's heart rate increases above 160 bpm (beats per minute), cardiac output may start to decrease because the heart does not have enough time to fill with blood completely before it contracts again. As for the effect of temperatures, slight hyperthermia (elevated body temperature) can result in an increase in heart rate and strength of contraction, whereas hypothermia (lowered body temperature) can slow the heart rate and strength of contractions. Considerable hypothermia can even cause the heart to stop beating.
Thus, with proper water temperature that avoids hypo- or hyperthermia and hydrostatic pressure that does not excessively change venous return, the heart rate is more likely to decrease, making 'B' the correct answer to the question of what happens to the heart rate in those conditions.