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 and used.
Personal Floatation Device (PFD)
Life jacket or vest doesn't necessarily apply as a name since many are now inflatable somewhat like the ones on airliners, but much sleeker. 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. Jeff Chan got a Mustang Survival 4-Pocket Vest. The Stearns Versatile is a roughly equivalent foam industrial vest, and there are many other styles of pocketed vests, including inflatable ones. Some of the nicer ones are intended for Search And Rescue "SAR" work.
Especially in waves or in the dark, it's really hard to see people in the water. A whistle can help rescuers locate 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.
Strobe or flashlight
Strobes usually attach to the vest and can be seen for about 3 nautical miles. Essential at night. Many are automatically activated. Some of the newer strobes are starting to use LEDs instead of flash tubes. LEDs can be more durable, simpler, brighter, more efficient, can also act as flashlights, etc.
An earlier-generation Adventure Lights LED strobe got high marks in the BoatUS Crew Over Board evaluation, which is a major civilian exercise in ocean rescue. The latest-generation Adventure Lights VIP Survivor Light SOLAS model uses a newer LED, is much brighter, and claims 5 nautical mile visibility, which exceeds flash tube strobe range, and seems plausible given the very high intensity of LED light. (Adventure Lights appears to be the maker of the infrared strobes widely used by the U.S. military, so their lights are probably built to a high specification.)
As of Fall 2009, these new lights aren't in widespread distribution, but Night Gear carries them at a slight discount. Use the promotional code "Seastead" to get an additional 10% discount. (The author, Jeff Chan, has no connection with either company except as a customer.) Even with the discounts, the price is still pretty high at about $100, but this appears to be a very high quality and effectiveness light. It can mean the difference between life and death.
Handheld marine transceiver
A handheld radio that transmits on national and usually international marine radio frequencies.
First aid kit
The minimalist Adventure Medical Kits ".5" is waterproof and fits in a large pocket of the Mustang 4-Pocket Vest.
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.