How can I hide 'mark sharp' lines? Right now I'm working with a mesh, and have several lines that I've marked sharp. Now I'd like to mark seams to unwrap it, however the cyan 'sharp' lines are always on top of the red 'seam' lines, preventing me from seeing my unwrap progress.

How can I toggle the display of marked edges?
Yes. While in Edit mode, have a look in the 3D View's Properties region (N). There's a panel labelled "Mesh Display", and within it, a section labelled "Overlays". There you have an array of checkboxes to toggle the visibility of various overlays. If you want to disable sharp lines, left-click the Sharp checkbox and you should be good to go.