There are obvious body doubles for both Christina's and David's nude scenes. You can see their faces clearly - most notably, Christina's double's face can be seen through the tree's leaves when she emerges from the lake. David's double's face is clearly seen during the sex scene with Christina.
Prof. David Ash: Miss Webb, I have been to hundreds of houses, countless seances, and I have never yet seen something that can't be explained in rational terms. The people I've met all have one thing in common; pain.