A necessary piece safety gear for kayak fishing open water is a Marine Very High Frequency (VHF) Radio. Personally I have had great experience using a Standard Horizon Handheld HX209 VHF Radio.
The MTI Dio F Spec PFD has a built-in pocket for the handheld VHF. 
Along with the VHF radio I like to use an external microphone/speaker. The radio fits inside a zippered pocket designed with a built in antenna port allows it to protrude through the pocket. Attatching the mic to the shoulder strap of the PFD easily allows monitoring the radio along with keying
up to speak. The external mic is submersible which works best for us kayak anglers. The speaker/mic also has a clip making it easy to keep on my shoulder area. it is necessary to access the radio only when switching channels and changing the volume.
Marine Radios vs Cell Phones:
Marine radios are the primary device for on water marine communication including distress. Cell Phones are primary land based communication devices that contact land based telephones. So they both have their own uses. A Marine Radio is able to be heard by anyone in ranged dialed into the same frequency. A cell phone is a person to person connection which will not enable assistance from other vessels in the area.
That being said, they are not comparable since they both do different things. A VHF Marine Radio is best for on water use due to ability of multiple vessels hearing your transmission. This greatly reduces arrival time for assistance.
HX290 Features
- Floating
- Submersible IPX8 (1.5 meters or 4.92 feet for 30 minutes)
- Slim form fitting case
- Selectable 5 and 1 Watt transmit power output
- Supplied with a 1140 mAh Li-ion battery, AC and DC chargers and a charging cradle
- Glow-In-the-Dark strip
- Huge channel display
- Optional speaker microphone or headset capable
- All USA, International and Canadian Marine Channels
- Programmable Scan, Priority Scan and Dual Watch
- NOAA Weather channels and Weather Alert
- Key lock
- Squelch level on display
- Low battery indicator
- Preset key used to recall up to 10 favorite channels
- Optional FBA-42 Alkaline battery tray available
- 3 Year Waterproof Warranty
Great features of this model VHF radio are the Key Lock for in pocket use. The additional external mic ability. Rechargeable Li-ion battery. Weather alert.
Manufacturers website http://standardhorizon.com/indexVS.cfm?cmd=DisplayProducts&ProdCatID=85&encProdID=43176D11CC8DF3B49149D8AF658C0CC3&DivisionID=3&isArchived=0
