When Tolkien arrived at this analysis of the exordium I do not know. What is clear is that these lines opened up before him as an Oxford undergraduate and that he pondered them for the rest of his days. That is a lot of pondering by a mind prone to both scholarship and storytelling. And there is more than one story that arose from Tolkien's pondering of the horizon of the shoreless sea.
Consider Eärendil. A very good account of Tolkien's Eärendil is in Mythlore 1991, by Carl F. Hostetter, who observes that Tolkien's "mythology was shaped around, and grew from his vision of Eärendil: 'And thus did all the fates of the fairies weave then to one strand, and that strand is the great tale of Eärendil' (BoLT II)." Hostetter identifies five dimensions of Eärendil: the Star, the Messenger, the Eagle, the Mariner, and the Herald. Of the Mariner, Hostetter says:
This aspect of Tolkien's Eärendil, together with that of the Star, is the most pervasive, and can be traced from the earliest poem to Tolkien's final writings on this myth.
A full discussion of the significance of the Sea and Mariners to Tolkien lies beyond the scope of this paper. It may be briefly noted that everywhere in Tolkien's fictional writing, and often in his professional writing, the sea serves as a metaphor of longing for, and separation from, a lost, unfallen state (e.g. in the essay Beowulf: the Monsters and the Critics: "But from the top of that tower the man had been able to look out upon the sea." (M&C)
While the association of Eärendil with the sea was for Tolkien a deeply personal one, there are, to be sure, good philological precedents for this association. There are several legends about Eärendil in which the sea figures. In Saxon Gramaticus' Historia Danica we find...
Does anyone reading this see what is going on? Hostetter's philological analysis is systematic and thorough, and yet it is no different - please forgive the comparison, my dear - to talking with Aiks. Of all keys to the ancient northern mythology, Beowulf is the oldest, and obviously the most dear to Tolkien's heart. Yet the philological discussion is always directed to the later northern stories and the Old English poem that was Tolkien's great scholarly study is vanished.
I am not being quite fair, because Hostetter's analysis is confined to etymological readings of the proper name 'Eärendil' so he has no cause to mention Beowulf. But it is rather pertinent context.
It is really curious because, if you consider the above, the allegory of Beowulf is invoked, with the Anglo-Saxon artist said to be able to look out upon the sea. But somehow the allegory has lost its target. The Anglo-Saxon artist and the great Old English poem have disappeared, and the view from the tower, rather than taking us into the exordium to the poem, becomes a token of Tolkien's private and personal idea of literature, or something that means that we are not to look on it.
Hostetter sees everything bar Beowulf, and Beowulf is at the center of things. Put aside everything else except the exordium, the first 52 lines that tell the story of Scyld Scefing, and consider the young Tolkien caught by these lines, caught by an imagination of those who dwell beyond the sea and what they may still mean to those who dwell on mortal shores, an imagination of passages beyond the horizon, mortal sailors who arrive at the further shore of the shoreless sea. And now picture him reading the famous lines 104-5 of Christ I:
Eálá Earendel engla beorhtast
Ofer middangeard monnum sended.
Hail Earendel brightest of angels,
over Middle Earth sent to men.
