Possibly Water Scavenger Beetle Larva from Japan we believe

Subject:  Mystery Bug, Japan
Geographic location of the bug:  Northern Hyogo Prefecture, Japan
Date: 06/22/2019
Time: 07:13 AM EDT
Your letter to the bugman:  I found the strangest bug out on my run today. We’re in the middle of the rainy season right now (late June), and a massive downpour had just finished. I live in a pretty rural area and I found this guy on the road next to the rice paddies. At first I thought it was a caterpillar of some kind, but the way it was moving was a little off. Instead of the normal perstaltic motion it was kinda flopping around more like “the worm” dance, raising it’s head pretty significantly at the end of each movement. And when it flipped over while crawling I was surprised to see it had six legs! The skin looked pretty soft and covered in silt, and combined with the fact that it wasn’t very elegant moving around on land I guessed it was probably aquatic. When I got home I googled pictures for dragonfly larvae though, they don’t match at all! It was about 10 cms long, with a rather big and fat “tail”, six small legs, and small but noticable mandibles. What kind of bug could this be? I’ve never seen anything like this in the three years I’ve lived here. Thank you!
How you want your letter signed:  Justin

Dear Justin,
Had you only provided us with a dorsal view, we might have pondered this being a Soldier Fly pupa, but the legs and mandibles rule out that possibility.  We believe this is a Beetle larva.  We will continue to research this identification while having posted it as Unidentified.

Update:  Cesar Crash believes this is a Water Scavenger Beetle larva in the family Hydrophylidae.