When gettin' in a boat, there ain't no way ta turn about or backwater.
Typically ya do this by usin' the "A" or "D" keys fer turnin' about an the "S" key ta backwater in the boat, which is often used in slowin' down or makin' a full 180° turnabout.

Instead, these controls are completely unuseable since them Horse mechanics were added in 1.6, makin' the boat only able ta turn if yer movin' forward at the same time.... and there ain't a way ta stop it once it's goin'.

Ta reproduce this,
• Get in the water with a boat.
• Get in 'said boat.
• An try ta turn 'round or backwater while yer not movin' forward.