Why a river berth would complicate Mikasa's preservation

Why not move Mikasa up a Japanese river into fresh water?

Suitable river locations were densely populated, and the navy wanted to keep the ship in an area it controlled.

Moving Mikasa upriver is suggested as a way to place her in fresh water. The answer points to dense settlement around the stretches large enough to accommodate the ship. The navy would also want a preservation site that remained under its control. Those constraints complicate a move even if fresh water appears attractive.