To explain, in the movie, Marlin and his friend Dori are on the search for Marlin's son, Nemo. When they are at a loss, they just keep swimming on.
Then Dori starts singing, "Just keep swimming. Just keep swimming. Just keep swimming, swimming, swimming. What do you do? You swim, swim."
It might be an annoying song if someone kept singing it over and over, but really it's a good lesson in life.
Just keep swimming.
Just keep moving.
Keep putting one foot in front of the other.
I notice often that when I am sluggish, the more I laze around, the less I want to do. When I am extremely busy, I get home and am still in a tizzy, so I continue to be productive.
If you stop swimming, sometimes it can get hard to start back up again.
Whether this relates to a hard point in life or just getting motivated, it's usually a good idea to just keep swimming.