Jalalayn
His are the crafted ships [that sail] in the sea [appearing] like landmarks, like mountains in their tremendous size and height.
Ibn Kathir:
(And His are Al-Jawar Al-Munsha'at), meaning the ships that float,
فِى الْبَحْرِ
(in the seas), Mujahid said, "Whatever ship hoists a sail, it is from Munsha'at, if it does not hoist a sail, it is not from the Munsha'at.'' Qatadah said, "Al-Munsha'at means created.'' Others said that it is Al-Munshi'at meaning, "launched.''
كَالاٌّعْلَـمِ
(like A`lam.) This means, they are like mountains with their great size, and it also refers to the trade and commercial services they make possible, transporting cargo from one area to another and from one province to another. Ships provide various benefits for people, including transporting different types of goods they need. Therefore,
فَبِأَىِّ ءَالاءِ رَبِّكُمَا تُكَذِّبَانِ
(Then which of the blessings of your Lord will you both deny)
Tafhim al-Qur'an
"His are the ships...": Ships became possible only by His power. It is He Who blessed man with the capability and skill that he may build ships for crossing the oceans; it is He Who created on the earth the material from which ships could be built; and it is He Who subjected water to the laws by which it became possible for the mountain-like ships to sail on the surface of the surging oceans.
Here, the aspect of the blessing; and bounty is conspicuous in alaa', but the explanation given above shows that the aspect of power and good qualities also is present in it.
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The surah then mentions the ships that sail through the sea, some of which are very large, looking like mountains:
"His are the lofty ships that sail like floating mountains through the seas." (Ayah24)
The surah refers to these floating ships as belonging to God. They indeed sail by God's will. Nothing keeps them afloat in the sea, through its waves and currents except God's care. Hence, they belong to Him. They remain among the great favours with which He blesses human life. They provide a means of livelihood, transportation and comfort that people cannot deny.
Hence, the comment:
"Which, then, of your Lord's blessings do you both deny?" (Ayah 25)