Our teacher has waited so patiently for our report but we have not submitted it yet which underlined word functions as a coordinating conjunction in the sentence

The coordinating conjunction in the sentence is "BUT". A coordinating conjunction is a word that joins two items of the same rank. Those items can be words, phrases or sentences. In this case,"but" is used to introduce the second statement ("Our teacher has waited so patiently for out report") which is different from the first one ("We have not submitted it yet"). The conjunction "but" presents a contrast between those two ideas.

adding more details about the struggle rainsford experiences as he swims to the shore