i would prefer to use cash to set up my sign and sail rather than have 350 tied up on my card.if i can use cash how much do i need to deposit when setting it up??

It'll depend on the length of your cruise, but if you know that the reccommended deposit for a card is $350 for your length of cruise, then that's what they'll recommend for cash as well. You'll likely need at least $200, no matter what.

No, you can use cash or travelers checks for your onboard account.

If you prepaid your gratuities on a card, then they will be charged to the card you paid them with. After that, you can still set up your sail & sign with cash if you preferred.