Close, but no cigar. (Spoilers)
I've seen Hell in the Pacific several times over the years. I really liked it the first time because Lee Marvin and Toshiro Mifune are a couple of my favorite actors, and the basic story of two WW2 combatants marooned on an island in the South Pacific makes for an interesting situation. The location and cinematography were very good also.
In subsequent viewings though, I've found too many errors and unrealistic aspects of the story which just don't make sense, severely detracting from the overall believability of the film.
1. Both actors were too old to play the characters; Marvin was 44 (but looked older) and Mifune was 48 when the movie was made. Realistically, they both should have been in their early 20's at the oldest.
2. Both actors had neatly trimmed beards through most of the movie, when the only cutting instrument available to them was Marvin's KA-BAR (knife).
3. When they first became aware of each other's presence, most likely one would have killed the other (but then there wouldn't have been much of a story).
4. Since they didn't kill each other, it doesn't make sense that they would harrass and torture each other just for kicks. It seems more likely that, once they realized that they could survive better as a team, they would have cooperated more.
5. Both before and after they became friendly, Marvin never made any effort to secure water or food on his own. He just kept stealing it from Mifune and lounging on the beach, playing with hermit crabs. If he was supposed to be a Naval Aviator, he would have been trained in survival skills.
6. Neither made any effort to learn to communicate verbally with the other. They just kept yelling at each other in their own languages.
7. The "This is MY log" scene was absurd and not even funny. (Marvin peeing on Mifune however, was funny.)
8. While the two of them are paddling the raft through the surf of the surrounding reef, Marvin is rowing with a single oar on the left side of the raft while Mifune is propelling the raft from the rear with his oar in a "stern sweep" (fish-tail) motion. This would have had them going in right-hand circles.
9. While on the raft at sea (at night), Marvin hears a plane flying overhead and starts blowing his whistle to attract attention. If he's an aviator, he'd know that he was wasting his energy.
10. In the last scene, they are getting drunk in an abandoned building at night, when they hear the sound of an incoming naval gun projectile. Immediately thereafter, an exterior shot shows the building exploding... in daylight, followed by the ending credits.
These are only the most glaring inconsistencies which for me, make for a disappointing movie. It could have been much better.