Seeking a short story about sea monsters attacking island colonies on an ocean planet. A space ship that comes to help and creates mutated monsters to attack the sea monster.
I believe that this is probably the Haviland Tuf short story "Guardians" (1981) by George R.R. Martin. First published in Analog, it was included in the anthology Tuf Voyaging featuring the adventures of Tuf.
The thread of the set of short stories is that the protagonist, Haviland Tuf, has come into the possession of a "Seedship," a 30km-long bio-engineering and bio-warfare ship from the Ecological Engineering Corps of the defunct Federal Empire of a previous era. Styling himself an "ecological engineer and a specialist in biological warfare" and successor to the EEC, Tuf flies about trying to make enough money to keep his ship going, and satisfying his curiosity as he does so.
Hearing that the planet Namor is not present at the Six Worlds Bio-Agricultural Exhibition because of sea monsters, Tuf goes there to satisfy his curiosity.
"Sir," he said. "I am the Ark. I am Haviland Tuf, captain and master here, crew entire. You are troubled by great monsters from the depths of your seas, I have been told. Very well. I shall rid you of them."
The planet of Namor is mostly water-covered and lightly inhabited:
Namor is a young planet, with neither the population nor the resources for the sort of struggle we have been plunged into. Three million Namorians are scattered across our seas, on more than seventeen thousand small islands. Another million huddle on New Atlantis, our single small continent. Most of our people are fisherfolk and sea-farmers.
First they are attacked by huge twenty-limbed monsters that destroy ships:
Then the first tentacle came curling up from beneath the ship.
It rose and rose. It was longer than the dredging claws. Where it emerged from the dark green sea, it looked as thick as a big man's torso. It tapered to the size of an arm. The tentacle was white, a soft slimy sort of white. All along its underside were vivid pink circles big as dinner plates, circles that writhed and pulsed as the tentacle curled over and about the huge farming ship. The end of the tentacle split into a rat's nest of smaller tentacles, dark and restless as snakes.
Up and up it went, and then over and down, pinioning the ship. Something moved on the other side, something pale stirring beneath all that green, and the second tentacle emerged. Then a third, and a fourth. [...]
Twenty tentacles had attached themselves when the ship abruptly listed to starboard. Survivors slid across the deck and into the sea. The ship tilted more and more. Something was pushing it over, pulling it down. Water sloshed across the side, and into the open hatchways. Then the ship began to break up.
Next come the flying monsters that attack aircraft:
Then, just ahead of the silver-blue Spirit of Aquarius, there was motion in the water, something stirring beneath that dark green veil. Something big, but not a dreadnaught. It was dark, not pale. The water grew black and blacker in a great swelling patch, then bulged upward. A great ebony dome heaved into view and grew, like an island emerging from the depths, black and leathery and immense, and surrounded by twenty long black tentacles. Larger and higher it swelled, second by second, until it burst from the sea entirely. Its tentacles hung below it, dripping water, as it rose. Then they began to lift and spread. The thing was fully as large as the airship moving toward it. When they met, it was as if two vast leviathans of the sky had come together to mate. The black immensity settled atop the long silver-blue dirigible, its arms curling about in a deadly embrace. They watched the airship's outer skin tear asunder, and the helium cells rip and crumple. The Spirit of Aquarius twisted and buckled like a living thing, and shriveled in the black embrace of its lover. When it was over, the dark creature dropped the remains into the sea.
Finally monsters come for the land:
It was a still picture, this one, not a tape. Haviland Tuf and Guardian Kefira Qay had a long time to study the dead thing where it sprawled, rich and rotten. Around and about it was a litter of human corpses, lending it scale by their proximity. The dead thing was shaped like an inverted bowl, and it was as big as a house. Its leathery flesh, cracked and oozing pustulence now, was a mottled grey-green. Spread on the sand around it, like spokes from a central wheel, were the thing’s appendages—ten twisted green tentacles, puckered with pinkish-white mouths and, alternately, ten limbs that looked stiff and hard and black, and were obviously jointed.
Tuf fights back with creatures from many worlds:
For the next twenty days, Haviland Tuf and Kefira Qay crisscrossed the dangerous skies of Namor in a painstaking grid pattern, seeding the seas. The Guardian flew her runs with elan. It felt good to be in action again, and she was filled with hope as well. The dreadnaughts and fire-balloons and walkers would have their own nightmares to contend with now—nightmares from half-a-hundred scattered worlds.
From Old Poseidon came vampire eels and nessies and floating tangles of web-weed, transparent and razor-sharp and deadly.
From Aquarius Tuf cloned black raveners, the swifter scarlet raveners, poisonous puff-puppies, and fragrant, carnivorous lady's bane.
From Jamison’s World the vats summoned sand-dragons and dreerhants and a dozen kinds of brightly colored water snakes, large and small.
From Old Earth itself the cell library provided great white sharks, barracuda, giant squid, and clever semi-sentient orcas.
The planet fights back:
"Fire-balloons sighted again off Mullidor Strand—hundreds of them. I can hardly give credence to these reports, but they say the lashtail mantas just carom off them now. Do you..."
"Men-of-war again, can you believe it? We thought they were all nearly gone. So many of them, and they are gobbling up Tuf’s smaller fish like nobody’s business. You have to..."
"Dreadnaughts spraying water to knock howlers from the sky..."
Finally Haviland, having decided that the attacks must be guided by an intelligence, goes searching for it:
Haviland Tuf frowned down at his muddy hands, and wiped them primly on his greatcoat. Taking Dax from his pocket, Tuf deposited the lethargic black kitten on the table. Dax yawned and stretched, and ambled toward the nearest of the Lords Guardian, who stared in horror and hurriedly inched her chair back a bit. Shrugging out of his wet, muddy greatcoat, Tuf looked about for a place to put it, and finally hung it from the laser rifle of one of his escort. Only then did he turn back to the Lords Guardian.
"Esteemed Lords Guardian," he said, "this is not dinner you see before you. In that very attitude lies the root of all your problems. This is the ambassador of the race that shares Namor with you, whose name, regrettably, is far beyond my small capabilities. His people will take it quite badly if you eat him."
Tuf forces the humans to recognize the intelligence of the natives, and their right to be not-eaten, and they in turn will allow the humans to fish the other creatures of the seas, and live on the land.