While performing the swimming animation (holding left control while underwater) placing a block always swings the right hand. This ignores what hand the block is in (main- or off-hand) and it also ignores the "main hand" setting in the Skin Customization settings. The right hand always swings, not the left.

This was tested in both 1.13.2, and the latest snapshot. (18w44a)

Below is a screenshot depicting the issue. A hastily-drawn arrow points toward the right hand, which is swinging even though the block of dirt that I placed is in my left hand.