What could be a possible explanation for the

length of a giraffe's neck?

Due to adaptation and evolution, the giraffes' neck has evolved to be extremely long. As we know, giraffe's tend to eat leaves from higher up branches in the trees. Since few mamals are able to reach up that high, they have more food.

To put it simply, a giraffe's neck has evolved to be so long due to their food choices.

they float because they are less dense than the water itself

something denser than water, like a rock, will sink to the bottom