I pretty much agree on Amp34's take on it. Anything that gives you the picture
you want. The wideish end of things might be more typical for seascapes, and the 18-55 IS kit lens might well be good enough. I doubt you'd need anything as fast or expensive as f/2.8 glass. Another option might be a pre-owned 17-40/4L, but Canon's 10-22 or something similar from another brand, while more extreme, might be a better choice for the vision you have in your own mind. You'll probably be stopped down somewhat anyway, so even a modest lens might be adequate. A lot depends on how large you want to print/display and what sort of final quality you are looking for. Probably as important as the lens choice is your technique and shooting style. Will you be using a tripod, mirror lockup and remote release, or are you happy to go with a more relaxed approach?
It might be worth scouring any landscape/seascape shots in this forum -
http://www.talkphotography.co.uk/forums/forumdisplay.php?f=35 and seeing if you can view the EXIF details to find out what sort of focal lengths, apertures, and perhaps specific bodies and lenses people are using. Remember to consider the impact of the crop factor when comparing shots from APS-C vs APS-H vs full frame bodies, and making judgements about focal length choice.
Just had a quick squiz and found this "seascape" -
http://www.talkphotography.co.uk/forums/showthread.php?t=213261 - 450D at 43mm and f/5.6.
Here's are a few of mine, nothing to get excited about, but just some ideas of focal lengths and apertures for "seascapes" of sorts....
- 40D hand held at 55mm, f/5.6 -
- 40D hand held at 22mm, f/8 -
- 40D hand held at 250mm, f/11 -
In other words, there is no "right" answer. There are many solutions. You need to know what you want to accomplish with the picture, focal length being the primary driver, but quality being a factor too, and how much you are willing to pay.