This is a subset of MC-14521, but I'm creating a separate bug for it because I understand the lack of keyboard steering is intended behavior, and I'm not sure this is.

In 1.5 and lower, it was possible to stop a boat by holding down the 'S' key. This was very useful, in particular when approaching land in order to slow down to a near stop to avoid destroying the boat. The only way to slow a boat now is to spin 180 degrees around, looking directly behind you, and holding down 'W'. Obviously, this makes it impossible to tell how far you are from obstacles in front of you, making it unnecessarily difficult to safely approach land. Combined with the fragile nature of boats, this change makes boating extremely frustrating.

While no one would accuse Minecraft of attempting to be realistic, it's worth noting that in a real boat, you would never spin around in a rowboat to slow down. Instead, you drag your paddles or paddle backwards, actions which are modeled well by holding 'S'.