Very High Frequency (VHF) Radio is the standard marine communication system. It is also referred to as Marine VHF or Maritime Radio Telephone.
If you wish to obtain a radio to use for casual communications between kayakers, then consider using FRS and GMRS radios - they are inexpensive and FRS is unlicensed. Marine VHF is not a toy and is not intended to be used for casual chatter.
Many other boats and all commercial fishing boats are equipped with a VHF and the commercial boats are required to monitor channel 16, the International Distress and Calling frequency, all of the time they are on the water, except when broadcasting on a specific frequency. The Coast Guard monitors channel 16, this is the channel to use if you are in distress. The Coast Guard also monitors Channel 22A as their working channel. A VHF is both an attention getting and a location marking signalling device. It can be used to call for help and the Coast Guard can home in on the signal. Note that in some areas close to the US border, all marine VHF traffic is monitored and tracked - vessels crossing the border while transmitting are noted.
The frequencies assigned to marine VHF are listed under marine frequencies.
Channel 16 is the standard "hailing" frequency. Use this frequency when you just want to talk to someone for a while. Get their attention on channel 16 and then switch to a working channel to continue with the conversation. In the US, Channel 9 is also available as a hailing frequency for recreational boaters. It can be used to free up channel 16 for more important hailing.
If you are interested in buying a marine VHF handheld, consider the list of marine VHF features available today.
Consider the Marine VHF licensing requirements as well.
If you are using the radio, ensure that you conform to the standards for terminology and communication. Some of them are covered in Marine VHF protocols.
A few abbreviations you might see used in marine VHF articles:
PTT - Push To Talk
Rx - reception
Tx - transmission
WX - weather