Pocket Edition, private world, peaceful mode. I was in an ocean biome approx 2000 blocks from my bed (respawn) which I had already explored and mapped but I had returned home to grab an extra conduit to make looting shipwrecks and treasure hunting easier. When I returned I was swimming (no boat) with the dolphins towards an island where I had left some stuff in a chest. Some of the chunks between me & the island were a bit slow to load but since I knew the area I just kept swimming towards the island. All of a sudden without any hearts lost, or damage taken sounds, it said I had died. No explanation of how I died and it wasn't possible that I had drowned since I wasn't underwater and as I said I went from full hearts to dead instantly.
I returned to the same spot near the island and picked up all my stuff but there was nothing in the area that could explain why I died. No magma blocks to drag me under, and being on private and peaceful: no players, no mobs.
This isn't the first time I wasn't patient enough to wait for all the chunks to load if I knew the area but never before have I died instantly so I'm only guessing that might be the issue.