Minor example; Momotarou's drop in performance during the Samezuka training sessions in episode 4 actually quite nicely illustrates his favorite proverb - "even Homer nods". "To nod" here actually does not mean the motion you do to say yes, but actually the one when you fall asleep, that is "nod off". The proverb means that even a genius (or an excellent athlete such as Momotarou) sometimes turns in inferior work compared to what they usually deliver.
As to why Rei sinks in the beginning when he tries to swim. He's fairly lean even compared to the other boys, and was part of the track and field team which focuses on building proper muscle to vault a pole and become slimmer to cut wind resistance. Both of which are detrimental to swimming. Muscle is denser than fat, and thus heavier, meaning that his muscled body is denser than the water so he'd have a harder time floating, and his low body fat means that there's barely anything there to keep him afloat. No wonder he sinks in the water.
In addition, the different in butterfly vs the rest of the strokes that he's unable to swim successfully. Freestyle, breaststroke, and backstroke all depend on some level upon the body being carried along by the force of the stroke through the water. Butterfly, by contrast, is a stroke where the arms and body are constantly moving in order to be propeled through the water properly. It's the one stroke that Rei's muscled body is well suited for, because it gives him the strength and stamina to constantly be propeling himself without stopping to float through the water.
If we take them separately, Makoto and Rin's backstories are already pretty traumatizing for a small child. Now if we look at them together we see that they have quite a lot in common: Both lost someone they cared about, Rin his father and Makoto the old man he was friends with, in a typhoon that sank their boat three kilometers off the coast. In episode 6 we see Makoto clinging to Haruka as a funeral part walks by and in episode 7 Rin is seen with Gou being part of one. Coincidence? I think not.
More like "Fridge Sadness" but, Makoto accidentally murdered his goldfish. Goldfish are supposed to live a decade at least but his died in a very short period. He obviously didn't know how to take care of them despite the fact he loved them.
There's also the fact that they were "festival prize" goldfish, which are often unhealthy. There is a significant possibility that they were already sick and he couldn't save them... which might be even sadder.