Bill
seven bells
Seven bells in a four-hour watch means 30 minutes before the watch ends. The mate wants Bill disposed of before shift change—pure administrative convenience.
holystone in a clout
A holystone is sandstone used to scrub decks—heavy enough to sink a body. A 'clout' is a rag or scrap. They're wrapping Bill in literal garbage.
ten fathom beneath the keel
A fathom is six feet. Sixty feet down is shallow for ocean burial—Bill barely makes it past the ship's draft before hitting the weeds.
stand his wheel
Taking turns at the ship's wheel (steering). The crew's only concern is who covers Bill's work shifts. Not 'mourn' or 'miss'—just logistics.