Isn't "slow steaming" just what passenger airplanes do with cruising speed? There's an optimal point on the speed/fuel consumption curve that minimizes fuel us

Isn't "slow steaming" just what passenger airplanes do with cruising speed? There's an optimal point on the speed/fuel consumption curve that minimizes fuel use, and this is what ships are doing, isn't it? Or is there some other consideration for ships that makes the slow steam speed optimal beside fuel consumption?