The odds are 100-1 that your sinks drain overboard.
However, you need to trace every bit of plumbing on your boat from one end to the other...'cuz if something springs a leak or you should start taking on water, that's NOT the time to START finding out which ones can sink the boat and which ones won't.
As for your fresh water system:
Best way I know of to drain the tank: turn on all the faucets.
Recommissioning the system--which actually should be done at least annually--will solve your foul water problems. I've posted directions for doing that at least a dozen times...do a search for "Fresh water maintenance" or "Recommission fresh water system" to find 'em. If you can't, find 'em, I'll post 'em again.