(a) (i) Henry's law represents a relation between solubility of gases in liquid and pressure. Scuba drivers when comes towards surface, the pressure gradually decreases. This reduced pressure releases the dissolved gas present in blood and leads to formation of bubbles of nitrogen in the blood.
(ii) At high altitude atmospheric pressure is low due to which availability of oxygen is also low as compared to surface, which causes difficulty in breathing. In that condition we feel weakness and discomfort.
(b) Soda water bottle kept at room temperature fizzes on opening due to different pressure inside and outside the bottle. When the bottle is opened to air, the partial pressure of $\mathrm{CO}_2$ above the solution decreases. As a result, solubility decreases and hence $\mathrm{CO}_2$ bubbles out.