Robin400
The Marine Distress Frequency Channel,16 is 156.8 MHz. Early marine band VHF radios were all crystal controlled and the frequencies and mode of transmission (frequency modulation (FM)) was chosen to prevent interference to airband transmissions and also direct communication between ships and aircraft. Aircraft use amplitude modulation or AM and the two systems are technically incompatible. There was some military reason for doing that at the time, although the reasoning is lost in the mists of time.
When digitally synthesised equipment became available, more marine channels were slotted in between the existing ones to make old and new equipment compatible. Looking at the actual frequencies against a list of channel numbers makes no sense at all as the frequencies hop about all over the place as the frequencies go up from Channel 0.
Additionally, marine radio systems are capable of full duplex transmisson and reception on some channels to enable virtually normal telephone conversations to be carried out. These channel pairings require the transmitter and receiver frequencies to be as widely spaced as possible because the transmitter and receiver sections are operating simultaneously. Tight filtering and some clever technical work arounds are needed to allow the system to work. The same full duplex system also works and is used on marine short wave frequencies but is somewhat more prone to interference. Simplex transmissions are more usually used for this reason.
As far as airband systems go, selecting the correct channel or frequency is more critical now, but otherwise the equipment functions just as it did before.