No, its his character design.
Superman was created to be able to beat anyone and anything.
thats why every time he "loses" or "feels beaten" its purely psychological.
Superman as a character is not meant to be beaten, and any story where he is, is either a "What-If" scenario and non-canon or a writer that doesn't understand his character.

no, Superman is made to be able to overcome any challenge, not through training but through simply being superman. He is as strong as he needs to be.
He doesn't need to do anything to keep his power, he just needs to exist and he can save the day.
Doomsday vs Superman where doomsday kills superman has been retconned to never happen, because superman is not supposed to lose. Ever.

Why should he? he's the king of atlantis. He doesn't need to give a damn about most super villains who only threaten a city or two.
The only times where he really needs to do anything is when a world-scale calamity could happen that would directly affect the ocean in a negative way. and most of them affect it positively, in destroying entire continents or cities. He would just see it as "Hey, more ocean for me to rule over and less polution."