'The waters getting cold and the suns setting on what could be you're last night alive, the boat went under so fast you had barely enough time to don clothing. Now what do you do?'

Its a bad environment for survival, in fact being adrift at sea is possibly the most daunting of all survival situations. The sea is an inherently hostile environment due to a number of factors:

Oceanic Traits

Some areas of the worlds oceans, especially in the northern hemisphere can experience rough seas and weather the year round. Seas closer to the equator tend to be more forgiving with warmer waters and calmer seas, although seasonal storms such as monsoons, hurricanes and the like are occasionally present. The sun, although essential for life can be a double edged sword at sea, the suns rays can be inescapable on an exposed craft or driftwood. Added to this the reflective effects of a glass-like ocean and sun can take on a harsher, more deadly effect.
The water temperature of the worlds oceans can range from a deadly -1 C up to a scorching 34 C with a fall in temperature at night-time. The temperature range can vary with the seasons but as a general rule the closer to the equator the warmer the ocean gets.

The Human Body

Jumping into the sea from a stricken ship or vessel may be the quickest way from danger but can also be the quickest way into it as well. From a high platform or the upper levels of a cruise ship the impact on the water surface may be sufficient to knock you unconsious. Survival courses do teach a technique to prevent this, but even then it is a last resort, especially if there are alternative evacuation methods.
Another factor is that without a survival suit there is a risk in cold water of gasping for air on impact and drowning, so ensure you take and hold a deep breath before jumping. Survivalwise the human body can only survive for a certain period of time in sea water before succumbing to exhaustion / unconsiousness and drowning. Evolution has seen mankind evolve in a land-based environment and part of this has meant the human body operates at about 37 Celcius and will begin to lose body heat once immersed in anything below this temperature. Treading water will accelerate the cooling effect further. A wetsuit can slow down this process and a drysuit or survival suit will reduce the 'chilling' effects even more.
If there are several survivors good plan is to form a close circle to conserves body heat and can raise group spirits. This also makes it easier for search and rescue teams to locate survivors. Oceanic predators will usually avoid contact with humans, although any cuts and injuries should be kept out of the water if possible. Shark attack is rare and unlikely, but studies have shown that if the worst should happen, sharp blows to the snout of nose of a shark can deter it.

Lifeboats

It is in a lifeboat or even a makeshift life-raft that there is a greater chance of being rescued. Ideally the craft will have some kind of overhead shelter on board from the elements and be highly visible to rescuers. This could be anything from a ground sheet protecting against the sun to a hardened carbonfibre shell.

In the United Kingdom the term Lifeboat can mean the actual craft that the RNLI (Royal National Lifeboat Insitution) Rescue Organisation use. These craft are distinguished by the orange coloured paintwork on the hull. However for the purposes of this section assume that Lifeboat means that which survivors are on board.

Modern lifeboats have on board, as a minimum: Flares, water, first-aid kits, sea ink (attracting rescuers), fishing gear, heliograph mirrors, water purifiers, a sea anchor (slows down drifting from wreck to aid rescuers), life jackets and water. There may also be a rain-water collector to gather fresh water that way. There may also be a heater. Not all lifeboats have food due to the close proximity / attention of rescuers. Most ocean liners and craft far out at sea do however. Some of the latest lifeboats are nearly unsinkable by the elements thanks to advances in carbonfibre design and the fully enclosed 'submarine' hull. A further innovation in lifeboat technology is the escape craft evolution which is a system that launches a lifeboat at a 45 degree descent angle (Azimuth) away from a stricken vessel and is much quicker than the traditional 'davit and cable lowering' technique. Some oil & gas platforms and vessels have this system, but it's an expensive and not industry wide.
Attracting rescuers attention can be crucial as even on board a lifeboat poor weather can mean its difficult for even airborne rescuers to spot. Flares / Smoke dischargers are a valuable tool in a survivors arsenal and should be used wisely. They are normally located inside water tight storage containers on lifeboats. Proper care is needed when firing flares as the parachute flare can be lethal if used incorrectly. Hearing a rescue helicopter may not be enough to begin launching them, for best results (unless the weather is ruining visability) wait for the rescue craft to be at least within visual range ( 1 - 6 miles) before launching. Note that red flares are for day use and orange for night use (an easy way to rember is that street lights at night light up in sodium orange colour!) they could be hand flares or parachute flares. Use parachute flares initially to attract attention then ignite the hand flares, smoke, strobe flashers to (hopefully) give away your actual position to the rescue craft. The most advanced life boats may have an EPIRB (Emergency Position Indicating Radio Beacon) system on board that (when activated) sends out constant distress signals that rescuers can use as a beacon. Another useful accessory is a radar reflector or simply a large piece of metal (usually vertically mounted) that a rescue crafts radar system can pick up.

Life Raft

Similar to a lifeboat but slightly inferior is the Life raft, all offshore platforms and maritime shipping have these onboard. They are located close to the railings and normally inflate when 'rolled' off the side (via a bungee trigger) if a life boat is unavailable the next best thing (and possible quicker to deploy) is the Life Raft. It usually contains all the supplies a Lifeboat has but without an engine (although oars are usally onboard).

Should rescuers fail to appear or worse still the search is called off then the best solution is to try and make for the nearest land mass. If this isn't possible it might be best to try heading for international shipping lanes and pray for passing ship or vessel. Ration and conserve water as much as possible. Most modern lifeboats have limited propulsion so ocean travel is possible although fuel is a limiting factor. Water is much more important in a survival situation than food; as a person can only survive about three days without water in comparison to three weeks without food.

Survival At Sea | Man Overboard | Stormy Sea