Watch the The Sea Generation online guitar lesson by Jason Loughlin from Essentials: Surf Guitar
For our first section, the melody is on the D string. The open G and B strings sound as the 5th and 7th against the C chord and b7 and 9 against the Am chord.
The second melody we play over this progression is on the B string. I'm using the open E string to drone against the melody. The last section over the G to Am keeps the melody on the B string and our droning E string. All of these melodies resolve to chord tones at the end of each phrase.