A good 5-6 watt VHF’s range is up to 5 miles over open water if you are in the kayak. In practical terms more often 2-3 miles to reach another kayaker or small boat. This is further reduced by being actually in the water or if land masses or other structures block line of site to the receiving station. Range is basically line of sight. The ability to obtain help can be greatly increased if there is a lot of large boat traffic around (in theory any boat that has a VHF is required to monitor VHF channel 16 at all times) or like around around the Chesapeake Bay the USCG has a extensive array of tall towers. A VHF is only to be used for marine use. It is not legal to use it like a walkie-talkie on land.
A DSC capable VHF radio typically has a single protected button, that when pushed will send out a constant distress signal with position. If properly registered the USCG will also have your name, type of boat, and contact information at a minimum. Once pushed you must contact the USCG or local 911 system by voice or VHF radio to cancel the distress call. Turning off the radio does not stop a response. Bear in mind, that unless there are other boats nearby, it can take the USCG an hour or more to get help on site. This goes for any distress signaling system.
Beyond this there are satellite based communication devices. These are generally able to be used anywhere in the world and do not normally have a range imitation as long as you have a clear view of the sky. All of these are required to be registered like VHF radios with DSC. Registration is usually free.
The most basic is the PLB or personal locator beacon. These can only send a distress signal and usually can not send messages or allow the cancelation of a distress signal once activated. This is not limited to maritime use. Some can send non-emergency location information.
Beyond this, and much more expensive are satellite messenger systems. In addition to the above devices, these allow sending of text messages, and in some cases receiving them You can generally cancel a distress signal. These require a subscription as well as registration, which can be fairly expensive. They can have, depending on how much you pay, a wide range of other abilities. These might include advanced GPS abilities, two way text or voice communication, non-emergency two way communication and location information, other radio bands including VHF, strobe light, etc.
In addition to the above is the EPIRB (emergency position-indicating radio beacons. Unlike the devices above, an EPIRB is registered to a vessel rather than a person and is only for maritime use. These are pretty limited in function but a fairly expensive due to USCG requirements they must meet. They generally come with a strobe mounted on top and must be waterproof and float upright. They are generally more powerful that the devices above and must be able to transmit for a minimum of 48 hours continuously. Some are designed to begin transmitting automatically when in the water and released automatically if a boat sinks. There is a long list of technical specifications these must meet to be certified. They tend to be heavy, bulky and very expensive compared to other emergency distress signaling devices. They are not really suitable for a kayak.
Some people, in addition to a VHF, carry a PLB if they are in remotes areas or inland areas with no USCG coverage.