"The wind shifts, they've run aground upon the sandbar! Circle and shoot!"
- Rangi Bat-Erdh, a captain of the Outrigger Thousand
Once, a vast empire rolled down out of the steppe to the music of horse hooves beating in the millions. The extent and influence of this empire can still be felt in the present, though the exact details of its heyday are lost to obscurity, nonstandard forms of history-keeping, and a couple of propagandistic chronicles written after the empire's fall by scholars of their surviving foes, for good measure. Still, it had been a spectacular force in its day, its victories and achievements known the world 'round. Its failures, too, were just as significant.
One such failure was when the empire began to overstep its more distant boundaries. Rather than opting to engage in a more economic war or a good game of politicking, it wanted a grand foe neutralized. From across the sea. Yes, they had access to auxiliary forces and through vassals possessed a fairly large navy. But the horses of their most elite troops took up much space and many resources, and the general leading the expedition was too proud and too nervous of falling behind his fellows in the perpetual game of winning favor from the khagan to step down or reign things in. And then the storms came.
The armada was scattered and utterly smote by what was surely a mixture of blundering lack of foresight plus divine providence. At least, that is how it is remembered. The invasion was never kicked off, and many never came home. But not all died out on the open seas. There was an archipelago, closer to their foe's homeland than their own, and the survivors who drifted there found that they had to lay low, lest foolhardy counter-fleets and patrols seize them or sink them. Waiting and then returning home wasn't too likely, either. Poor luck ensured that few auxiliary mariners and even fewer ships survived intact. The horsemen of the open steppe were trapped upon a series of several dozen small, tropical islands in the middle of a sea of water instead of grass. But the ingenuity which had gotten their people that far did not fade in the face of staggering odds. Indeed, it only amplified it.
Bow and arrow remained each warrior's faithful companion, and grew only closer to his (or her) heart in the absence of their beloved horses. Most had been lost in the storms, and the few who remained could do little for their former riders. Fortunately, the numbers of mares compared to ungelded males allowed for them to remain a small yet constant population. Reduced in size now and unable to bear the weight of a rider and all of his gear, these beasts are a reminder of a time ages past, kept as symbols of status rather than as one of the bare necessities of life.
The nomads' own population persisted despite a few initial logistical and genetic hurdles, and they intermingled with their former subjects as well as a few of the sparse locals to ensure a new and diverse people's survival. Fish, crabs, "herds" of placid manatees, and other sea life have replaced traditional staple foods of sheep and cattle. The warm weight of fermented mare's milk in one's stomach has given way to the lighter but sometimes more inebriating kick of wines and spirits made from fermented fruits. Their heavens-worshiping faith was hindered in some respects, but given a boon in others, for the clear seas around them are grand mirrors for the eternal blue sky.
Instead of saddles, the descendants of the nomads strap themselves into rigging of catamarans and surprisingly swift and fierce war-dinghies. Instead of holding their breath with each bowstring draw to anticipate the lift of hooves in a gallop, they wait for the thrust of the waves beneath, or a turn in the wind whipping through their sails. Their fierce independence remains as strong as ever, and when they sail from island to island or beyond in small fleets, they do so prepared to cast any interlopers down upon the coral reefs of their homewaters. More than a few have conflicted with others on what it means to follow the "old ways", and the pirates who set out from their stock can be terrors of the waves indeed. Others have pursued trade just as the caravaneers of old did, and now their garish currencies of cowry, nephrite, and pearl can be found in markets across the coasts of the eastern and western continents.
(I can't rightly take credit for thinking this whole thing up. The credit of inspiration, as well as a very different and fascinating take on nomads pulled out of their element, goes to the backstory of Fallen London. Check out their Khanate faction in the games Echo Bazaar or Sunless Sea sometime, if you can avoid dying of course.)