If you do get the 24-105 I am sure it can handle a lot of what you want to do at the beach, but, you may also want to invest in a lens in the 10-20mm range (Sigma or Canon, but the canon is prefered if you have the money). There are some shots that you may want to get were you need the wide angle lens for.

That is the beauty of the 17-55... it is very sharp, fast, and wider. However, you do need to be careful with the lack of sealing at the beach (bad weather is great for beach shots)

I find my 24-105 handles most everything I want for landscape on a crop body, but there are a few times when my 10-20mm comes in handy. When I first got the 10-20, I did not use it much, but I find myself using it more and more.

Pat