Garner: "Pronunciation experts have long agreed that the word is pronounced /si-zem/, not /ski-zem/ or /shi-zem/" (note, those letter e's are supposed to be upside down; that special HTML character doesn't exist in ASCII ISO 8859-1).
That disagrees with's suggested pronunciation, but Merriam-Webster agrees with Garner in its preferred pronunciation.
A tidbit for the next time you're at a loss for conversation.