In order to be truly poignant, a tragedy must be sad in a whole myriad of ways. So, Jaime's potential death on Game of Thrones would be awful because, aside from Cersei losing her lover and Tyrion losing the one family member he can stand, Jaime's death would mean Cersei never learns who really killed Joffrey. Of course, there are other tragic consequences, too, like the loss of Nikolaj Coster-Waldau's incredible bone structure from the show; the fact that we never got to see Jaime and Brienne of Tarth fall in love with each other; and the gaping absence of Tyrion and Jaime never getting a final conversation before the older brother's demise, if he has indeed drowned. But Cersei never learning that Olenna was behind her son's poisoning would matter.
This was stressed by the fact it was mentioned again at all. After all, the audience already knew the Queen of Thorns was behind the death when, in Season 4 Episode 4, Olenna more or less told Margaery she had poisoned Joffrey: "You don't think I'd have let you marry that beast, do you?" Eagle-eyed viewers may have already picked up on Olenna playing with Sansa's necklace at Joffrey's wedding (one of its crystals, speculated to have contained poison, later went missing). So having Lady Tyrell repeat this confession to Jaime prior to her own death felt significant — and not just as an epic pre-mortem clapback.
Olenna's actions were significant in terms of the larger story, since this was the death that tore the Lannister family apart. When Joffrey pointed at his uncle who was examining the goblet of wine following his nephew's strange symptoms, this led Cersei to assume that her younger brother was behind her son's death and order his arrest. While the youngest Lannister was never the biggest fan of his family, it was this that led him to run away and support the Targaryen bid for the throne. If Cersei never discovers that her brother wasn't the one behind her son's death, presumably this would fuel her malice if she ever managed to capture Tyrion. We might even get a torture scene equal to that of the pink lipstick and the Sand Snakes.
But, according to dramatic conventions, the very fact that Olenna confided that it was her who murdered Cersei's son to Jaime suggests that the younger Lannister twin won't drown. There's very little that takes place in Game Of Thrones just for the sake of it. Russian playwright Chekhov once said “One must never place a loaded rifle on the stage if it isn’t going to go off. It’s wrong to make promises you don’t mean to keep.” While Game Of Thrones is more ambitious than many plays in terms of scope, it does seem to follow this one basic dramatic rule.
Joffrey's poisoning was a good example of this. In Season 4 Episode 1, Ser Dontos gives Sansa a necklace in a scene that doesn't appear to have any connection with anything. But, as we later see, it's this same necklace that contains the vial of poison that Olenna uses to murder Cersei's son. Presumably, Lady Tyrell had asked Ser Dontos to deliver it to the elder Stark sister.
Realistically, it is the scene between Olenna and Jaime, above all others, that suggests that Jaime will be permitted to live a little longer; at least until he delivers the true identity of Joffrey's murderer to his lover. What unfolds then is anyone's guess.