Imagine a scenario where three friends--Jamar, Renata, and Hiroshi--are standing at the edge of a calm lake. Jamar, who is quite tall, wades into the water first, and the water reaches up to his waist. Next, Renata, who is of average height, walks in until the lake comes up to just below her shoulders. Finally, Hiroshi, who is the shortest of the trio, joins and the water level reaches his chin. To play a game, they all submerge their heads under the water simultaneously. The question arises, from the perspective of an observer on the shore, what parts of each person's body will be visible above the water when they are standing still with their heads submerged? Also, considering their heights and positions in the water, how would each friend be able to identify the presence of the others while their own heads are under the water?
When Jamar, Renata, and Hiroshi submerge their heads under the water, the parts of their bodies that remain visible to an observer on the shore will differ based on the depth at which they have waded into the lake. Since the water reaches up to Jamar's waist, the upper half of his body from the waist up will be visible, including his arms if they are not also submerged. For Renata, whose water level is just below her shoulders, only the very top of her shoulders and her arms will likely be above the waterline. Hiroshi, with the water reaching his chin, will have no part of his torso visible, and maybe only the top of his head or hair might peek above the surface, provided that he doesn't dip his head fully beneath the lake's surface.

Without the use of their heads and eyes above the water, each friend would have to rely on alternative senses or indicators to deduce the presence of the others. They might feel water displacement generated by movements nearby if any friend decides to move, noticing the ripples or currents that reach them. Additionally, they might hear sounds and vibrations through the water, which propagates sound efficiently. Any splashing or movement that's not in sync with the natural lapping of the water against their bodies may suggest another person's whereabouts.

However, if they remain still and there is no sound, discerning each other's precise location would be challenging, as the clues available through senses other than sight would be minimal. They could presume the presence of their friends at their last-known positions unless new sensory information suggests otherwise. If a friend moves and displaces water, creating a distinctive ripple or sound, the others might then infer that someone is indeed moving within the water, albeit without precision concerning the mover's identity or specific position.