a material that stretches 1% to failure (like steel/aluminum) can ballpark bend to a radius 100 times the thickness. so a 1 meter cable could bend 100m radius before cracking. assuming 10x margin that would be 1 km radius. large but not crazy. A tube that size can easily span 1 km trenches in water. you could also add a few meters of foam around it to make it neutrally buoyant and just barely press on the ocean floor.