Section 28. All coastwise vessels not sailing under register and not carrying oil, hazardous material or hazardous waste in bulk as cargo in or entering upon areas of special interest within the waters of the commonwealth, United States public vessels, and United States fishing vessels shall be exempt from compulsory pilotage; provided, however, that if any such vessel requests the aid of a pilot, he shall, when permitted by the laws of the United States, serve the same in like manner as vessels not exempt, and shall be entitled to regular compensation.