I believe that any prep that softens the hairs enough to cut without tugging can work. I used to shave with SE disposibles immediately after a shower, shampoo, and conditioner using water only. I had to work fast before the hairs dried. Using lather is better, but I still rinse my face for final clean-up using water. I get my closest shaves and smoothest skin by straight shaving with uberlather, DE followup using uberlather, and DE cleanup using water only. Water only works OK for me if I wash the hairs well using soap or shampoo before using water. The DE works well after the straight because there is not much left to cut and the straight is gentle on the skin when used correctly.
I recommend gaining proficiency with a DE before trying a straight which has much steeper learning curves for use and maintaining a shave ready edge. I also recommend a trip to the local pharmacy or supermarket for a cheap VDH boar brush and a puck of VDH soap before trying water alone. Even a can of foam will probably give a better shave than water alone.