فَأَنجَيْنَاهُ وَأَصْحَابَ السَّفِينَةِ
Then We saved him and the Companions of the Boat,
means, those who believed in Nuh, peace be upon him.
We have already discussed this in detail in Surah Hud, and there is no need to repeat it here.
وَجَعَلْنَاهَا ايَةً لِّلْعَالَمِينَ
and made it (the ship) an Ayah for all people.
means, `We caused that ship to remain,' whether in itself, as Qatadah said, that it remained until the beginning of Islam, on Mount Judi, or whether the concept of sailing in ships was left as a reminder to mankind of how Allah had saved them from the Flood.
This is like the Ayat;
وَءَايَةٌ لَّهُمْ أَنَّا حَمَلْنَا ذُرِّيَّتَهُمْ فِى الْفُلْكِ الْمَشْحُونِ
وَخَلَقْنَا لَهُمْ مِّن مِّثْلِهِ مَا يَرْكَبُونَ
And an Ayah for them is that We bore their offspring in the laden ship. And We have created for them of the like thereunto, on which they ride. until;
وَمَتَاعاً إِلَى حِينٍ
and as an enjoyment for a while. (36;41-44)
إِنَّا لَمَّا طَغَا الْمَأءُ حَمَلْنَـكُمْ فِى الْجَارِيَةِ
لِنَجْعَلَهَا لَكُمْ تَذْكِرَةً وَتَعِيَهَأ أُذُنٌ وَعِيَةٌ
Verily, when the water rose beyond its limits, We carried you in the ship. That We might make it an admonition for you and that it might be retained by the retaining ears. (69;11-12)
And Allah says here;
فَأَنجَيْنَاهُ وَأَصْحَابَ السَّفِينَةِ وَجَعَلْنَاهَا ايَةً لِّلْعَالَمِينَ
Then We saved him and the Companions of the Boat, and made it an Ayah for all people.
This is a shift from referring to one specific ship to speaking about ships in general. A similar shift from specific to general is to be seen in the Ayat;
وَلَقَدْ زَيَّنَّا السَّمَأءَ الدُّنْيَا بِمَصَـبِيحَ وَجَعَلْنَـهَا رُجُوماً لِّلشَّيَـطِينِ
And indeed We have adorned the nearest heaven with lamps, and We have made such lamps missiles to drive away the Shayatin (devils). (67;5)
meaning, `We have made these lamps missiles, but the lamps which are used as missiles are not the same lamps as are used to adorn the heaven.'
And Allah says;
وَلَقَدْ خَلَقْنَا الاِنْسَـنَ مِن سُلَـلَةٍ مِّن طِينٍ
ثُمَّ جَعَلْنَـهُ نُطْفَةً فِى قَرَارٍ مَّكِينٍ
And indeed We created man out of an extract of clay. Thereafter We made him a Nutfah in a safe lodging. (23;12-13)
There are many other similar examples