[kaya-ooyook] 'He who rolls up with his paddle in his kayak after a capsize
Critical situations and Call for assistance
In general you should not hesitate to call for help as you usually will not get charged or prosecuted in any way.
Every rescue organisation stands for better calling one time too often than one too less!
How to decide that a sitation cannot be handled alone any more?
This is very difficult and many case studies show that emergencies develop slowly and are often not recognized! 
They also show that usually several things go wrong and alone they would not have been a problem in the first place, so have a sharp eye on multiple happenings!
How to signal for help?
There are several methods of getting help and attention:
Usually it is a good idea to always take your mobile with you in a watertight bag.
Nowadays you have a GPS included and some helpful apps for emergency situations.
The big disadvantage of course is that you cannot rely on a network connection on the sea and that you cannot reach nearby craft directly.
To have a seagoing radio on a seakayak is probably one of the best, but most expensive possibilities as you will need a licence to operate it, the radio itself and yearly costs.
But you get the possibiliy to reach nearby craft directly and talk to them.
Of course the range can be very limited in cold, windy and wavy conditions as a handheld version is quite weak.
(Stands for Emergency position indicating radio beacon)
This is a small device that also is quite expensive and needs registration.
However once activiated it will send your GPS coordinates directly to an emergency association who will immediately initilize a rescue operation for you (after they checked with given contacts if the emergency might be real).
These shoot small magnesium flares 300m high in the sky where they will sink down slowly on small chutes and will be visible for 1-3 minutes.
Best way to make your position and emergency situation clear to any visible crafts.
Usually you will need a special license to buy, store and use these.
Often free to buy, relatively cheap and easy to use, these handheld smoke (day) or red (night) torches will signal your position.
Of course only effective on short distances.
Water colour
Another way to mark your position is to use floating, bright green colour. 
Local rescue associations will be able to track you down on the way the currents distribute the colour accross the water surface.
Most effective for aircraft and only in daylight.
Whistle and other methods
If you don't have any of the above signal devices, you should at least have a whistle although (if not special types) these will already be useless with quite low winds and are often mistaken for gulls, wind or other common noises.
When swimming, you can also attract craft by strongly kicking up the water surface with your legs.
Picture showing left to right: Flares (red), EPIRB, Radio, towing line, water colour.
 (c) 2015 Qajaujuq All rights reserved