Allah U says ﴿وَإِذْ﴾ which means, ‘and when’, to refer to a specific time frame in which the incident happened.
﴾فَأَنجَيْنَاكُمْ﴿“so We saved you” from drowning, and it was a calamity that did not take place, and if Allah U had said, نَجَّيْنَاكُم “we saved you” it would have meant that they were indeed drowned but later saved.
In the previous verse, the torture happened, so the term used is نَجَّيْنَاكُم “we saved you”
and here ﴾فَأَنجَيْنَاكُمْ﴿ “so We saved you” is used as the calamity did not strike.
The word ‘Bahr’ means, sea. It generally refers to salt water seas, and could sometimes refer to rivers. Allah saved the children of Israel and drowned Pharaoh by the same means. And none is able to do this except Allah U. He is able to save and destroy by the same means.