Personal safety equipment
Equipment that can help you survive in the water, particularly the open ocean, is pretty well known. All should be waterproof to a high standard. Some of this equipment overlaps with life raft equipment. This list may seem excessive, but these are commonly regarded as standard safety equipment for operating on the ocean. Many lives would be saved if they were more widely carried.
Personal Floatation Device (PFD)
Life jacket or vest doesn't necessarily apply as a name since many are now inflatable like the ones on airliners. Inflatables are compact, light and comfortable but more expensive. They may be a little less durable than foam life vests. Many inflatables deploy automatically. Some don't. Conventional foam live vests (non-inflatable) are less comfortable, but are much less expensive and generally offer more convenient pockets for carrying some of the items below.
Especially in waves or in the dark, it's really hard to see people in the water. A whistle can help locating someone, assuming the user is able to use it.
In daylight, a signal mirror can be used to signal rescue aircraft. A really cool one made by ACR has a red hologram used as an aiming device, a little like a red dot firearm sight. The hologram lets you see where the reflected light is shining. The ACR Hot Shot is sold in a kit with a whistle for around ten dollars.
Flashlight or strobe
Strobes usually attach to the vest and can be seen for 3 to 5 nautical miles. Essential at night. Many are automatically activated.
Handheld marine transceiver
A handheld radio that transmits on national and usually international marine radio frequencies.
First aid kit
EPIRB or PLB
These are satellite transmitters that send your position to international rescue organizations like the Coast Guard. The ocean is really large and finding one person on it can be nearly impossible. Having a device that transmits your position continuously improves the likelihood of rescue greatly. PLBs have already saved the lives of many stranded/injured hikers, including in remote parts of Alaska. In other words, they work on land too. They probably work in more locations than radios since they communicate with space-based satellites, not ground- or aircraft-based radios. For anyone on the open ocean an EPIRB or PLB should be considered essential, at least if you want to be found.
A foam suit or waterproof clothing system that covers everything except your face. Essential in waters outside the tropics, since hypothermia can kill you in in a few hours. Somewhat similar to a diver's drysuit, a survival suit can replace a PFD.
Generally works world wide. Expensive.
Signal flares, smokes and marker dye can be useful for attracting help or pinpointing your location to rescuers when they're close enough to see them. Flares and smokes are required on most water vessels. Some are small enough to carry.
Some things not included
A cell phone may work near large cities or seasteads, but is generally useless on the high seas. A FRS or GMRS radio may be more useful for short range communications within a group.