The Silver Menace by Murray Leinster

Excerpt:
Nita turned around and smiled at him. The car was rather crowded with seven people in it. Gerrod was looking curiously at a bit of the silvery jelly, with which he had filled several pails before leaving the yacht. He took a bit of it between his thumb and forefinger and rolled it back and forth speculatively.
It seemed faintly granular to the touch, but at the slightest pressure underwent a change that felt like crumbling, and was nothing but watery liquid.
"I'll bet anything you care to name," he said thoughtfully, "that this is just a mass of little animalcules with little silvery shells. The silvery shells would account for the reflection we see."
"The captain of my yacht," observed Morrison, "said that he thought it was like a milk sea. That's a mass of little animals that glow like phosphorus in the dark."
"Perhaps," said Gerrod meditatively. "I'd like to look at this stuff under a microscope."
"Oh some of it will go to the government chemists," said Morrison with a large air, "and they'll figure out a way to kill the little beasts. There's a cure for everything."
"Perhaps," said Gerrod.
The plane flew on steadily, Davis finding some amusement in gratifying Nita's suddenly aroused curiosity about every part of the seaplane. When her curiosity about the plane was satisfied, however, and she began to make inquiries about himself, Davis was much less comfortable.
He tried to be evasive, but she pinned him down, and was filled with excitement when she found that he was the same man who, as Lieutenant Davis, had flown the two-seated flying machine that had destroyed the Black Flyer and with it Varrhus' menace to the liberty of the world.
She tried very hard indeed to get him to tell her the story of that fight, but he blushed and said there was nothing to tell. It would be hard to say to what lengths she would have gone had not something outside the plane caught her attention.
"There's the horizon!" she exclaimed. "We've come to the edge of the silver sea, and from here on it's just the plain, good, old-fashioned ocean."
The line that marked the point where sea and sky joined was indeed visible, and a gradually widening bank of darker blue showed that the silver sea had indeed come to an end.
As the seaplane flew onward the darker, wave-tossed ocean came toward them and passed below, but blended so gradually with the jellied ocean that it was impossible to tell where the silver sea ended and blue water began. It was evident that the silver sea was still growing.
Then, for a long time, the seaplane sped onward over the blue waters, while Nita tried ingeniously to extract from Davis the details of the fight with the Black Flyer.
Davis was acutely uncomfortable, but nevertheless he felt strangely disappointed when the dim line of the coast appeared ahead. He hovered a moment to get his bearings, and then sped northward toward the aviation station to which he was attached.
Nita, too, seemed disappointed. She had enjoyed tormenting Davis, and he impressed her very favorably. After the plane had swooped downward and come to rest on the water a scant two hundred yards from the hangar in which it was kept, she turned to Davis.
"Well," she announced, "since I haven't been able to make you tell me what I want to know this time I warn you I shall make you tell me next time."
Davis smiled.
"May I hope there will be a next time?"
Nita smiled at him.
"I shall be angry if there isn't," she said demurely.