Britta has never swum before, but it comes to her instinctively.  It's pretty much like running in slow motion while holding her head up out of the water.
Waves slap her in the face.  She snorts the water out of her nose but can't do much about what gets in her mouth.  Not without dropping her precious burden.
The boat is 50 yards away.  40.  30.
Salt stings her eyes.  Trickles down her throat.
20 yards. 10.
She goes under, fights for the surface, and she's there.  Claws gripping the rubber of the little boat.  Climbing.
Made it.
Britta drops her cargo, spends exactly three seconds catching her breath, and leaps back into the sea.
In the bottom of the boat, a bedraggled clump of orange and white fur sneezes, staggers to its feet, and opens a pair of tiny blue eyes.
"Meee?"

(Blah blah stuff happens)

The sea beast gulps, shudders, then thrashes wildly as crimson shoots from its blow hole.  Its mouth opens and a great torrent of blood and flesh spews out.
From amid the bloody detritus, Britta watches the beast try to swim off, convulse, stop moving, and sink.
Together, they drift down...down...down.
Britta is proud of her last battle.  She killed a sea beast from the inside with only her front claws and teeth.
She kept (number) humans, two other cats, a dog, and a What The Fuck Is That alive for (time) in the Silent World.
She produced, cared for, and gave a chance at a good life to two strong, healthy daughters.
Not bad for a feral-born runt with a reproductive condition.
Kitten number 3 is drifting down too, still struggling, silhouetted in the dwindling light above her.  It's the little black runt with the missing leg.  Too bad she can't save him, but that's life.
That's death.
Sorry, kid.
Suddenly, there's a disturbance at the surface.  A big black cat plunging in.  His sleek, muscular limbs propel him downward, golden eyes blazing in the murky sea, searching until he finds the kitten.  He grabs the little one's scruff in his strong jaws and his eyes meet Britta's.
Go now, she silently tells him.  Goodbye.
The last thing she sees in this life is Aku reaching the surface with their son.  Then her vision goes dark and the world dissolves.
A twinkle appears in the blackness above.  It splits into two, then three dancing points of light.  Closer, closer, until it resolves into two winged lionesses towing a chariot.
Britta greets the Valkyries with dignity and pride.