A theory on how Luke did not die
From rewatching this movie, just now on Netflix, I noticed he said specifically "I had to literally go through the depths of Tartarus" to get the sarcophagus. Could he have drowned from the first movie and then came back because he wanted to help Kronos? (i.e. Kronos, or someone in cahoots, saved him in order to help himself/Kronos) For those who think he got eaten by Poly, could a similar thing happen? (Horcruxe esque, I know, but its possible).
For the latter, I like to personally think that as per the close up look after he lands and Poly says "just in time for dinner", he fought his way out. Like he said earlier in the film, "Turns out you're not the only Half-Blood who is hard to kill". Would he really be defeated THAT easily? A similar thing happened to Baxter Stockman in the 2003 Ninja Turtles cartoon, where at the end of one episode (Hun on the Run, where Hun goes to rescue Karai), they REALLY made it look like Stockman was destroyed by Shredder, but at the beginning of the next episode, he looked no different. This is also evident by Shredder telling Hun "You live" after completing his mission, considering both him and Stockman dead weight and useless, hun only proving himself useful by saving Karai Check out the ending here, at 20:43
https://www.youtube.com/watch?v=T5T1W0CWmfc