In order to answer this question, one must first understand how a dolphin breaths. Dolphins, like humans, are mammals that have a set of lungs for respiration. This requires dolphins to come to the surface of the ocean to breath when then need oxygen. This is different from fish, who utilize gills to filter oxygen from water for breathing. The gills of fish allow them to directly remove the necessary oxygen they need from water while swimming, and therefore makes it unnecessary for them to surface. In the case of dolphins, and many other mammals residing in water, they are unique in that instead of being able to breath through their mouth or nose, they make use of a blowhole located on their upper backs that is separate from their mouth. Because of the need to breath oxygen from surface air, and despite being able to hold their breath for periods of time much longer than other mammals such as humans, dolphins can still drown if they are not able to return to the surface for oxygen. A common reason for this is getting caught in fishing nets. Once caught, the dolphin is trapped underwater without oxygen which eventually leads to death by drowning, in a similar way to if a human were trapped underwater in a fishing net. Hope this helps!