Most model boaters have experience with hulls that are shaped to slide through water easily. A 20Kg item, possibly shaped like a brick, will need a lot more power than something designed to pass through water. If the water is moving, it must be capable of propelling itself faster than the water is moving, and will need to continuously travel at water speed to hold station. Displacement hulls have a maximum economic speed in water - look up "hull speed".
20Kg? There have been several underwater camera projects on various sites, Even from several years ago, I don't think any complete projects were that heavy and cameras have become smaller and more capable.