Signs in the Oceans, Mountains, Rivers, Roads and Stars
Allah tells us how He has subjected the seas, with their waves lapping the shores, and how He blesses His servants by subjecting the seas for them so that they may travel on them, and by putting fish and whales in them, by making their flesh permissible to eat - whether they are caught alive or dead - at all times, including when people are in a state of Ihram. He has created pearls and precious jewels in the oceans, and made it easy for His servants to recover ornaments that they can wear from the ocean floor. He made the sea such that it carries the ships which plow through it. He is the One Who taught mankind to make ships, which is the inheritance of their forefather Nuh. He was the first one to travel by ship, he was taught how to make them, then people took this knowledge from him and passed it down from generation to generation through the centuries, so that they could travel from country to country and from place to place, bringing goods from here to there and from there to here. Thus Allah says:
In the last verse (16), it was said: وَعَلَامَاتٍ ۚ وَبِالنَّجْمِ هُمْ يَهْتَدُونَ 'And (He created) signs. And through stars they find the right path.' Since trade-oriented travel was mentioned above, it was considered appropriate that conveniences which Allah Ta’ ala has provided in the heavens and the earth designed to help travellers cover distances and reach destinations be mentioned alongside. Therefore, it was said: وَعَلَامَاتٍ (wa ` alamat), that is, 'We have set up many signs in the form of mountains, rivers, trees, homes and things like that in order to help people find their way on the earth.' It is obvious that, had the earth been flat, there was no way man would stray around in his effort to reach his destination.
As for the last statement in the verse: وَبِالنَّجْمِ هُمْ يَهْتَدُونَ that is, the way travellers know their bearing through territorial signs, quite similarly, they also find out their orientation through stars as well. The modality of the address here seems to hint that the initial purpose of the creation of the stars is, for that matter, something else - however, along with it, their additional benefit is that they help in the charting of one's course when travelling.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 16–18
Here, two things are mentioned: maintenance of balance on the earth by raising mountains and the provision of signs to guide people on their way. Geographical studies show that when the oceans were formed, the earth started shaking. Thereafter, high mountains emerged on the surface of the land, and in this way as a result of the two-phased action, balance was restored on the earth. Had there been no such balance on the surface of the earth, human life here would have been impossible, or at least extremely difficult. Similarly, on his travels, the human being needs signs with the help of which he can make out the correct direction and reach his destination without going astray; here too the perfect arrangement is in existence. The human being of ancient times used to find his way with the help of such things as rivers and the stars. Nowadays, he finds his way with the help of magnetic instruments. Speedy flights on land and water as well as in space are possible with their help. Had signs of such a type not been in existence, human activities would have remained confined to a very limited sphere.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.