Flexible pvc tubing, thick wall. Density is about 1.4, along with the cable, so it will sink. Appropriately sized inside diameter for your wire.. ,... (8awg I assume because of the distance, but I would upsize, eventually they always want more, and the bigger the tubing, the less chance of a hook snagging, unless your fishing marlins).
Assume the lowest temp at the bottom will be 36° for contraction purposes.
Junction box at each end and as mentioned, if there is ever a need you can just pull it out.
If it needs maintenance, you can use any still viable conductor to pull a rope, or again, just pull it out and redo.
Our high school’s pond’s aerator/fountain is plumbed this way.