Originally Posted by Erin
General rule of thumb is to use the reciprocal of the focal length as the shutter speed. i.e. a 300mm lens needs 1/300 th of a second shutter speed to obtain a sharp picture. I'm not sure if your camera affects focal lenght by 1.5 or 2 times. In other words your 300 lens may be acting as a 450mm. In which case you need 1/450.
Might be worth trying it at 200mm. You may find the aperture is wider and allows a faster shutter speed to obtain correct exposure. The longer the zoom the 'slower' the lens (smaller aperture) and therefore the slower the shutter speed required. If you are really serious it would be worth picking up a prime lens with a fast aperture, say f2.8. Pricey though.
Very good advice there.
The canon 40D has very good low light performance for it's spec. Even ISO1600 can give reasonable quality. Going along the rules erin pointed out your camera has a 1.6 crop factor to the sensor so your getting 480mm when you have it set at 300mm.
Try setting the camera to ISO800 and use the RAW format (not Jpeg), spot metering, set the widest (lowest f number i.e f2.8) aperture that your lens can go down to. Then in manual mode set the shutter speed so the camera slighty under exposes the shot by say 1.5 to 2 stops(metering to the left of the read out).
This will give you the fastest shutter speed, will make sure non of the whites of the water will burn out and when you put the RAW file through a good RAW converter on your computer (the one that came with your camera isn't too bad) you can then mess about with the exposure while retaining the high lights and shadows.
You 40D is a very capable camera
If your using an IS lens then check out what way it actually compensates. Some lenses only compensate left to right movement or up and down. Not both and could make matters allot worse at sea.