Much obliged to everyone for your replies. I'm a newbie to this photography lark ! I'm amazed at how many different types of lenses are available particularly with the Canon EOS range of camera's.
I guess some lenses are interchangeable(?) between manufacturers. Am I right in thinking you don't have to stick with the same supplier ?
I say Canon because a friend recommends them highly although they do appear to be quite an expensive addition to the family !!
Many thanx again. 
You can't use Nikon lenses on Canon cameras (or Pentax or Sony, etc, etc) and vice versa. However, you can use third-party lenses, such as Sigma, Tamron or Tokina, on any DSLR as long as the lens has the correct mount for your camera.
If you want a DSLR you can't go wrong with Canon. They - and nearest rivals Nikon, who are also very good - have the largest range of cameras and lenses to choose from so all eventualities are covered.
Ken, I agree. I rarely use my 400mm lens for ships because, at that range, the image is often affected by haze and you end up with a weird mirage. As for shake, an alternative to IS lenses is using a monopod, fairly lightweight and not as cumbersome and space-hogging as a tripod.
