Yeah - my first thought was this thing was made to be mounted vertically. Probably going to turn out to be something simple and industrial related, and like 1rudeboy, it washed over and was probably presumed lost (obviously it’s lightweight - a good storm with a lot of wind could have taken something that lightweight and with such a large surface mass right off of a ship).
The rectangular flanges you mentioned are for attaching the legs to it. The bottom is conical shaped in order to ensure the ability to drain/empty it completely.