What Pyreflies said: I, and most others, do not mind the aspect of Tidus and Yuna getting it on at one point in canon. It's just how they initiated it that's bothering people on this board. Heck, remember when people were still talking about their kissing session at Macalania? There were rumours and implications that they did 'more' than just kissing. I think that they just put the fact that Kimahri was watching there just to balance out the two sides. 1: They had sex. 2: They just kissed. It's like that scene in FFVII where it was heavily implied that Cloud and Tifa had sex under the Highwind. Ambiguous moments like that are what get fans like us rolling.
But with the boat sex thing, it's probably the most OOC moment we've seen of Tidus this far in the novel. Remember: Tidus ALWAYS tried to protect Yuna from harm in some way. Sin attacking the S.S. Liki: First thing Tidus did was grab Yuna's hand. Moonflow: Tidus dove in immediately after Yuna. Wedding: Angry Tidus is freakin' angry. Etc., etc., etc.. Even in the Spring scene, he still seemed pretty protective of her while they were floating through the currents, what with the hand movements and all. Like I said earlier: If Tidus is supposed to be the far superior one out of the two in regards to boat/sea knowledge, the opposing person being his soul mate no less, then why didn't he moor the ship to somewhere in an ocean where there are frequent goddamn storms!? Tropical oceans are the most likely to have storms away from the shore! It's basic geography! I have to wonder, is Nojima trying to make Tidus as shallow and infuriating as possible? This is the guy who literally gave up his life to save his girl, and now he disregards their safety to tap Yunie's patootie. I know it's small when you compare it to everything else -coughblitzbombcough-, but it's still really annoying.
Of course, we all would expect a bit of realism, even in a Final Fantasy game. Mild realism, as displayed in this game, is the emotions of the characters and the common sense that they may or may not follow. Fantasy is a different world with often different laws, customs and species. It's not something in which you can make up any old rule to get out of something, or appear extremely bizarre for the sake of awe or wonder. It's partially why it's called Final Fantasy: Each game or 'entry' has a different world with different natures, as well as different threats and people. If FFX was REALLY realistic, Yuna would probably be really depressed, Tidus would end up in a mental hospital at some point, Kimahri would be walking on four legs, and the gang probably wouldn't last very long against fiends, which are basically mutated dead people, AKA zombies. For all we know, Tidus and Yuna getting along as soon as they are reunited could be considered normal in Spira's universe. If Wakka and Lulu could do it, why not Tidus and Yuna? XD
And Tresdias, in regards to the 'Bahamut created Tidus' thing, sorry for confusing you. We see throughout X that Bahamut very likely had a connection with him that was more special than any of the other fayth. In Dream Zanarkand, only Tidus could see him. The rest of the game, only Tidus and Yuna (and Auron) are 'allowed' to see his fayth form. (Of course, other summoners could as well, but Tidus was the only non-summoner that could at that point. Maybe the others could see Bahamut, but they didn't really address it.) He was the one who explained his and Dream Zanarkand's origins, and in X-2, you can ask him to bring Tidus back/initiate the Good Ending. Since the fayth are maintaining Dream Zanarkand, it could be possible that Tidus came from the portion of Bahamut's dreaming. The other fayth do have their own forms, which you can see when you revisit the temples after leaving Zanarkand. It's just Bahamut's fayth that appears to them for some reason.
But with X-2 and the whole 'Shuyin' thing... well, it is possible that Bahamut knew Shuyin and admired him. He and Lenne were popular icons in Zanarkand back then. Tidus is pretty much an imperfect copy of Shuyin's body with his own thoughts and memories.