Here is a more complete account of the battle from http://www.fordham.edu/halsall/ancient/arrian-bookVIII-India.html:
Thence they set sail and progressed with a favouring wind; and after a passage of five hundred stades the anchored by a torrent, which ,was called Tomerus. There was a lagoon at the mouths of the river, and the depressions near the bank were inhabited by natives in stifling cabins. These seeing the convoy sailing up were astounded, and lining along the shore stood ready to repel any who should attempt a landing. They carried thick spears, about six cubits long; these had no iron tip, but the same result was obtained by hardening the point with fire. They were in number about six hundred. Nearchus observed these evidently standing firm and drawn up in order, and ordered the ships to hold back within range, so that their missiles might reach the shore; for the natives' spears, which looked stalwart, were good for close fighting, but had no terrors against a volley. Then Nearchus took the lightest and lightest-armed troops, such as were also the best swimmers, and bade them swim off as soon as the word was given. Their orders were that, as soon as any swimmer found bottom, he should await his mate, and not attack the natives till they had their formation three deep; but then they were to raise their battle cry and charge at the double. On the word, those detailed for this service dived from the ships into the sea, and swam smartly, and took up their formation in orderly manner, and having made a phalanx, charged, raising, for their part, their battle cry to the God of War, and those on shipboard raised the cry along with them; and arrows and missiles from the engines were hurled against the natives. They, astounded at the flash of the armour, and the swiftness of the charge, and attacked by showers of arrows and missiles, half naked as they were, never stopped to resist but gave way. Some were killed in flight; others were captured; but some escaped into the hills. Those captured were hairy, not only their heads but the rest of their bodies; their nails were rather like beasts' claws; they used their nails (according to report) as if they were iron tools; with these they tore asunder their fishes, and even the less solid kinds of wood; everything else they cleft with sharp stones; for iron they did not possess. For clothing they wore skins of animals, some even the thick skins of the larger fishes.