2006 Mastercraft owners manual recommends that you use 15-40 in the v-drive unit and the zf hurth manual recommends ATF. Which is the correct fluid?

Most people use a 40W in the V Drive unit.

The V-drive is a Walters, ZF hurth is the transmission. My 05 V-drive said use HD30 oil and the transmission said ATF.

The ZF hurth is a trans & vdrive in a single unit! This unit had an issue where it was "noisy" when filled with ATF in warmer climates and so it was recommended buy the manufacture to use the 15-40w oil instead.

Ian is on the money. I run what MC calls for and use 15w40. Makes it simple that both the engine and trans run the same oil. Just make sure that you put what is already in there for the people right at the switch from trans oil to motor oil. If you do switch from one to the other, make sure you flush the system really well.