The safety gear that you'll be carrying will likely depend on what type of kayak trip you're considering, you may want just a few pieces of Safety Gear for a shorter day trip or for a longer or more isolated trip, you may want to have everything possible to keep trouble at bay or have a way to contact search and rescue in the event of serious circumstances.
For a group trip, not everybody needs to triple up on all the gear but if I was in the group I'd like to know who is carrying the Tow Line, Bilge Pump, Spare Paddle. Locator Beacon etc....
Here is some of the gear that I carry on the more out of the way trips. I shorten the list in the more populated areas but one of the minimum things that I always carry is a Cell Phone where it will get reception...
PFD and Whistle (Required in Hawaii State)
Paddle Leash (Heavy Duty) Many Kayaks also come standard with Paddle Clips. Paddle Clips are nice to secure your paddle while going snorkelling, fishing, snacking etc. but they don't do anything for you if you drop your paddle or the Paddle Clip breaks....
Water (more than enough for the planned trip) and snacks, energy bars or trailmix, something to quickly re-energize while still on the water.
Rashguard (I wear a heavy fabric rashguard (Skin Shirt) when I'm paddling in Hawaii for any length of time. It keeps my upper body from getting sunburned and it will also keep me warm even when wet if a situation deteriorates and you might otherwise be encountering hypothermia, yeah even in our warm climate it's possible....
Drybags or Dry Case (I carry my Cell Phone in a watertight Case, also my Locator Beacon even though it's waterproof. plus the case is a good place to carry a lighter, matches, knife etc.... But make sure that you pack the case where you can easily access it!
Emergency Repair Kit (for polyethylene kayaks) We now have some compact repair kits designed to be carried along as an essential piece of safety gear. The kit comes with it's own heat source to prepare the patch material which can seal pretty substantial cracks or holes
First Aid Kit - Size depends on group size and how many days you plan to be out
Cell Phone (if there is a chance of reception) We had one guy blown off the NW Big Island coast. No one knew he was out there and the high offshore winds were not allowing him to get back. He would definitely not have survived without his Cell Phone.... Even so they had a hard time finding him and he spent an extra night at sea. When they did finally track him down he was 120 miles out to sea. Doesn't happen too often but with a Personal Locator Beacon they would have come straight to him for a quick pick up, long before he got so far from land
VHF Radio - On trips where you are not always close to your friends, it's really nice to have the communication plus if you needed assistance you might be able to contact a boat in your vicinity instead of using a Personal Locator Beacon.
Bilge Pump - The larger kayaks usually do have hatches where you can use a Bilge Pump. Some smaller sit on tops have no hatch and no way to access the interior to use the Bilge Pump. You'd have to get the boat to land, or turn it upside down supported by another kayak if you had to get any substantial amount of water out of it. most kayaks can have a hatch installed if you'd like interior storage, plus it allows you to use a Bilge Pump.
Tow Line - For ocean it better be sturdy and at least 50'. Even so I've been towing in 8-10' seas and had the boat being towed, surfing down a swell and threatening to decapitate me. The longer towline also has more give and reduces the jerk in rougher conditions
Flares, Signal Mirror or signalling device
GPS - sometimes on longer trips it's nice to know how far you've got to go and what speed you're making. Plus you've got exact coordinates if you need to tell someone where you are.
Personal Locator Beacon - The nicer ones nowadays also include GPS coordinates when they are activated. These are really only to be used in dire circumstances. Activating one means you are needing immediate rescue.
Shark Sheild - Kayak Fishermen, having bait or blood in the water have the most likely chance of running across the larger species of sharks. The Shark Shield has a battery and wire that hangs in the water. If you see a shark, you flip a switch and a an electronic frequency is sent into the water. This frequency is not tolerated by the sharks and they will not get close. They immediately evacuate the area.
Personally I don't worry too much about sharks. Plastic is not the biggest attraction for them and in all the years that I've been paddling in Hawaii, I haven't seen any number of larger sharks that would cause me any concern.