I feel like i'm in a unique position to talk about this. My family has been involved in crocodiles in one way or another since the 1950's, from a close family being eaten, to working in their preservation and tourism.
Crocodiles are definitely a high level predator. Mangrove swamps make an ideal breeding and hunting grounds for pubestant crocodiles.
Each male crocodile will consider humans (and other animals) a challenge to its territory. When they have an opportunity to overpower a threat, they will do so and drown their prey and stashing the corpse under a log usually under water.
These humans would likely be forced to sleep in the mangroves, the British had control of the non mangrove areas forcing them directly into the territory of the crocodiles, the humans in the dark and asleep would EASILY be taken and drowned.
My uncle (deceased last year) tells me of the time when my family friend was taken by a local crocodile. She was knee deep in the water, splashing herself with the water on a hot new years night, the other party goers around heard a splash and not even a chance of a scream. Imagine this when you are 500 strong, in a thick area of crocodiles with mangrove swamp blocking noise.
I can't bring myself to watch narration on the story but ( https://www.youtube.com/watch?v=-s4lrLtMjwA ) is supposed to be pretty accurate. Its possible that I'm doxing myself and a clever person maybe can figure out my family from this.
This is one crocodile, with a territory size of about 200m. Imagine this across an mangrove area of the island, while you are hunted, tired and can't hear your team.
The worse part is, the japanese soldiers probably were not aware of the existence and the the sheer number of crocodiles in Ramree, it is a recipe for disaster. If the crocodiles didn't kill them, the bites and infections from them likely would.