I went to explore a shipwreck, and died with a full inventory. I went back to the shipwreck immediately, around 150 blocks east, and my items were nowhere to be found. No item entities were to be seen. None under the roof of the shipwreck, none floating to the surface of the ocean. None on the top of the ocean. I was back to the shipwreck within 5 minutes. The wiki says nothing about items despawning immediately, there were no lava blocks, no soul sand not magma blocks nearby.