I've used Les's method (multiple short exposures to make one long exposure) for some seascapes although I have used mental gymnastics to work out how many exposures to make! The method Les has suggested above looks much easier for my brain assuming I've got a calculator handy! Why I chose to use this method rather than the usual one long exposure is, in some circumstances (eg. waves flowing around a rock) in one 8sec exposure you might get 1 or 2 significant movements, where with the multiple exposures you can pick you slices of time and incorporate more movement in your shot. You can even time the shots so that other elements (like people, birds) aren't in any of the shots. The end result is definetly different, and something that needs to be experimented with.