I want to try to take some of those dreamy seascape pictures where the water looks like cotton candy during a sunset. My question is can I get the same effect with my 17-55 or should I buy a 10-18mm.
Either lens, creamy water will come from a 1/2 shutter speed or longer depending on the lighting and water movement. Essentially you are capturing the movement of the water. What you need is a strong neutral density filter which will bring down the shutter speed to work better with the aperture you want to use. The 10-18 performs quite well for the money actually and is clearly wider which is a plus if you like wide landscapes. I normally suggest the fixed aperture lens over a variable one, but not sure if the value is there in this comparison. The 17-55 you are paying for the 2.8 aperture which you likely will not use on those landscapes as you probably want plenty of depth of field. Its a good lens and better for other uses, but I do not know how it would fit with other lenses you may own. The IS is great to have if your shooting around 1/30 of a second, but long exposures on at tripod it needs to be turned off so not a help for those landscapes you are talking about. Also at the 500/600 price point you can start looking at full frame lenses which may be worth your investment long term if you upgrade the body down the line.. the 17-40 and 16-35 L lenses are great, built much better, and a better investment long term. Some food for thought at least.
I think that you should try your 17-55. But what you need is a ND filter and a tripod. Spend your money there for now. If you buy a new lens, you still need a ND filter and a tripod to get a slow enough shutter speed. Gary