Swimmer in pool - Fluid dynamics

I think the main mechanisme for the question a) is dead water but I dont know how to solve the following questions b and c. I tried to look at the internal waves but i was not sure how to use them because we need to derive energy loss. My instinct was that the person was the lengt of a wave but i dont know how i can use this and that he moves in a certain wave, an energy wave, because I think this is maybe not possible. Can anybody help me to know wich tools I need to use? Thanks in regards.

At the start of a swimming competition, the athletes try to stay under
water for as long as possible to limit the energy loss.

(a) What is the main mechanism that causes energy loss in surface
swimmers?

(b) Suppose there is an ideal swimming pool (where there is no
turbulence and friction behind a diver), how deep should a small
swimmer with a length of π/2 meters then dive to limit its energy loss
to 5% ? (HINT: Notice the curious length of the swimmer, and relate
this to a wave quantity.)

(c) Suppose the swimmer suddenly emerges in the middle of an Olympic
swimming pool 20 meters wide and 2 meters deep. How long will it take
for the energy to reach the other side? What approximation can you
make here?