At 419 words, this whopper may indeed be the longest sentence Twain ever wrote. Searching the internet for references to longest Mark Twain sentences, the best I found was this 262-worder from The Adventures of Huckleberry Finn:
The first thing to see, looking away over the water, was a kind of dull line; that was the woods on t’other side–you couldn’t make nothing else out; then a pale place in the sky; then more paleness, spreading around; then the rive softened up, away off, and warn’t black any more, but gray; you could see little dark spots drifting along ever so far away–trading scows, and such things; and long black streaks? rafts; sometimes you could hear a sweep screaking; or jumbled up voices, it was so still, and sounds come so far; and by-and-by you could see a streak on the water which you know by the look of the streak that there’s a snag there in the swift current which breaks on it and makes that streak look that way; and you see the mist curl up off of the water, and the east reddens up, and the river, and you make out a log cabin in the edge of the woods, away on the bank on t’other side of the river, being a woodyard, likely, and piled by them cheats so you can throw a dog through it anywheres; then the nice breeze springs up, and comes fanning you from over there so cool and fresh, and sweet to smell, on account of the woods and flowers; but sometimes not that way, because they've left dead fish laying around, gars, and such, and they do get pretty rank; and next you’ve got the full day, and everything smiling in the sun, and the song-birds just going it!
Another source refers to a mere (!) 236-word sentence as the possible record. If these claims to fame for sentences not much more than half as long as the one you've found can be taken seriously, then yours may quite possibly be the record.
As for the "story" behind these impressively extended and verbose sentences - clearly not just a one-off in Twain's writing, since there are several sentences scattered through his works which, although not as long as the example you've found, are nevertheless over two hundred words long and therefore pretty impressive and out of the ordinary - it may well be the case that the author was deliberately trying to create exceptionally long sentences for effect; partly, of course, longer sentences were simply more common in writing in those days, before the age of modernity, advertising and the internet, when people's attention spans shrank to the size of gnats' and it became necessary to impart information to them as swiftly and efficiently as possible before they went "TL;DR" and moved on to something more entertaining; partly, also, long sentences can be a practically useful writing device, as for instance when trying to include a great deal of description but make it feel compressed as though it's all happening at the same time (in your example, putting the entire description of the boat's motion and progress in a single sentence gives the reader the feeling that they're taking it all in at a single glance rather than reading about it piece by piece in a book; in the above description of a sunrise, again putting it all in one sentence is akin to compressing the experience into a single glance, the way it would be in the real world, rather than artificially extending it by a long wordy description); but in addition to their utility in writing, it may be that Twain was using these excessively long sentences for humorous effect or to make a point - a thesis supported, for example, by the following speech he once made in German on 21 November 1897, and for which he also wrote the entertainingly bad English translation which is provided below (a better translation is available here, but Twain's own has amusement value while still being comprehensible).1
I am indeed the truest friend of the German language - and not only now, but from long since - yes, before twenty years already. And never have I the desire had the noble language to hurt; to the contrary, only wished she to improve - I would her only reform. It is the dream of my life been. I have already visits by the various German governments paid and for contracts prayed. I am now to Austria in the same task come. I would only some changes effect. I would only the language method - the luxurious, elaborate construction - compress, the eternal parenthesis suppress, do away with, annihilate; the introduction of more than thirteen subjects in one sentence forbid; the verb so far to the front pull that one it without a telescope discover can. With one word, my gentlemen, I would your beloved language simplify so that, my gentlemen, when you her for prayer need, One
her yonder - up understands.
Before several days has the correspondent of a local paper a sentence constructed which hundred and twelve words contained, and therein were seven
parentheses smuggled in, and the subject seven times changed. Think
you only, my gentlemen, in the course of the voyage of a single sentence must the poor, persecuted, fatigued subject seven times change position! Now, when we the mentioned reforms execute, will it no longer so bad be. Doch noch eins. I might gladly the separable verb also a little bit reform. I might none do let what Schiller did: he has the whole history of the Thirty Years'
War between the two members of a separable verb in-pushed. That has even Germany itself aroused, and one has Schiller the permission refused the History of the Hundred Years' War to compose - God be it thanked! After all these reforms established be will, will the German language the noblest and the prettiest on the world be.
1 332 words.