Most VHF Marine Band radios used on small boats digital selective calling (DSC) radios with Class-D qualification. More elaborate DSC radios are qualified to Class-A requirements. To understand the distinction between DSC radios of various classes, see the UCG NavCenter website. A principal difference in a Class-A radio compared to Class-D is the capability to perform a distress alert relay. Installation of DSC Class-A radios is mandatory for certain classes of commercial and passenger vessels on a global basis. There are only a few VHF Marine Band radios with CLASS-A DSC features available.
The SAILOR 6222 radio is a Class-A DSC VHF Marine Band radio; it sells for about $1,500 to $1,750 at retail.
The ICOM GM600 radio is a Class-A DSC VHF Marine Band radio; it is not available for sale in the USA (most likely due to yet having been approved by the FCC). The retail price in US-currency would be about $1,800