A mariner’s whistle is a thin metal tube extending from a hollow ball:  it was used to command sailors’ attention at sea.  It’s a period charge, used as a badge by de Vere, Earl of Oxford, who had been Lord of the Admiralty, d.1513 [Coat of Arms IV(27), July 56, p.96; also Siddons II.2 303].  Post-period heraldic authors misinterpreted the badge as a wine bottle, but more recent research has corrected this.  The mariner’s whistle is fesswise by default.  See also musical instruments.