وَقَالَ ٱرْكَبُوا۟ فِيهَا بِسْمِ ٱللَّهِ مَجْر۪ىٰهَا وَمُرْسَىٰهَآ إِنَّ رَبِّى لَغَفُورٌ رَّحِيمٌ
"He said: 'Board it. In the name of God is its sailing and its mooring. My Lord is indeed much forgiving, most merciful.'"
In Arabic the verb rakiba takes a direct object: rakiba'l-safīna (he boarded the ship). Here the particle fī comes between: irkabū fīhā.
I record this as an observation about the word order, and I pass on the explanation the lexicographers record: the particle fī reports that the boarding is not onto but into. Not as one mounts an animal, that is, but as one steps inside a vessel.
The root ج-ر-ي: to run, to flow. The root ر-س-و: to be fixed, to cast anchor. Mursā — the place or moment of stopping; from the same root comes rawāsī (the firm mountains), treated across this series (al-Raʿd 13/3, Luqmān 31/10).
A variant reading is transmitted: it is recorded that the words are read majrāhā / mursāhā (with fatḥa, as nouns of time and place) or mujrāhā / mursāhā (with ḍamma, as passive verbal nouns of Form IV). On the second reading the sense becomes "its being made to sail and its being made to stop". Both readings are transmitted; I do not impose a preference.
And the place of the basmala here is worth recording. The basmala was treated in al-Fātiḥa. What belongs here is this: the places in the Qurʾān where the basmala is spoken inside an event, from a person's mouth, are few; this is one of them.
I record this as an observation: the sentence attaches both the motion of the ark and its rest to the same name. Neither its sailing is attached to Noah's skill, that is, nor its stopping to his decision.
The names cited at the moment the flood begins are al-Ghafūr and al-Raḥīm — not names of punishment or of power.
I record this as my own reading, and it is not binding: the sentence is spoken to those boarding the ark; the attributes cited for them are forgiveness and mercy. This is an explanation.