I’m a fan of de Camp’s short stories in particular, but I’m not entirely sure where he picked up that tidbit. It might derive from the Caliph Umar I ( r.634-644 ), the second successor to Muhammed:
*'Umar was a man of al-Hijaz who regarded the sea as a dangerous element; and as a good Moslem, following the Prophet and Abu Bakr, he refused to risk the lives of Moslems on expeditions that served no purpose. Only once did 'Umar himself order such an excursion: against the Abyssinians for reprisals for their attacks on the Arabian coast ( 641 ). This expedition sacked Adulis, but met with defeat on land. This confirmed the caliph in his caution.
…“But 'Omar dreaded the sea, and wrote to consult the 'Amr, who answered thus: ‘The sea is a boundless expanse whereon great ships look tiny specks; nought but the heavens above and waters beneath; when calm, the sailor’s heart is broken; when tempestuous his sense reel. Trust it little, fear it much. Man at sea is an insect on a splinter, now engulfed, now scared to death.’ On receipt of this alarming account 'Omar forbade Mu’awiya to have anything to do with ships: 'The Syrian sea, they tell me, is longer and broader than the dry land, and is instant with the Lord, night and day, seeking to swallow it up. How should I trust my people on its accursed bosom” Remember al-Ala. Nay, my friend, the safety of my people is dearer to me than all the treasures of Greece.’ "
This well-known story, combined with the landwardness of Arabic literature, has been responsible for the popular impression that thew Arabs have never been a seafaring nation. Conceived in the wide form, the impression is false, as can be seen from pre-Islamic history alone. What the story does show is that the northern Arabs at the time of Islam were not sea-minded…*
From Arab Seafaring by Albert Hourani ( 1951, Princeton University Press ).
At any rate, there is no Muslim ban on sailing or seafaring and indeed Islam was partially spread by sea ( particularly to Southeast Asia ). Numerous Muslim powers were also formidable sea-powers from the Caliphate ( post-Umar I, obviously ) right down to the Ibadi state in Oman, who expelled the Portuguese from their richest East African possessions north of Mozambique in the late 17th century ( and from which sprang the Sultanate of Zanzibar in East Africa ):
The men-of-war belonging to the sultan of Muscat had a formidable reputation in the Arabian Sea. In 1695 an Omani fleet of sixteen sail was sighted in Indian waters. The Muscat Arabs had begun to fit out heavy large warships during these years, no doubt to provide an effective force against European privateers who descended on the Arabian sea from the West Indies in the 1690’s. The French ship Legier, of forty guns, ran into two Muscateers, of sixty and eighty guns respectively, off Goa and was promptly engaged. The action and cannonade lasted until nightfall, and under cover of darkness the Legier managed to get away. She arrived in Goa in shattered condition; her captain had been killed.
From Trade and Civilization in the Indian Ocean: An Economic History from the Rise of Islam to 1750 by K.N. Chaudhuri ( 1985, Cambridge University Press ).