Displacement lag and thunder casting

If I wanted to use this sword am I going to constantly be hit by displacement lag?

If you're talking about the lightning part, it is not using the actual "Thunder" skill. It is its own skill.

Regardless, as far as I'm aware, the displacement lag for the Thunder skill only occurs while rafting, does it not?

It only occurs if your position changes between when it starts raining lightning strikes down and when it finishes, or in other words, during the skill animation phase. The raft just happens to help that happen.

If you do not change where you are relative to the sides of the river during that time, you wont get displaced.