"....." means that I replied with a couple of paragraphs of Canon-specific advice and returned later to realize I wasn't posting in the Canon Corner forum, that this was the main Underwater Photography forum. :doh: SB doesn't let you delete posts, so that's as close as I could get.
So let's try this again:
1. Which lens and camera body are you thinking about?
2. What would be the purpose of going this route? Are you trying to find a "good first lens" that will meet your primary needs (i.e. macro or underwater macro) until you can save your pennies to buy something better later for landscapes?
3. Are you aware of the limitations of macro lenses? When you mention landscape photography and "focused at infinity", are you trying to get classic landscape-type shots or are you just shooting from a distance?
Macro lenses are obviously designed for close-up shots, but the better ones will also do double-duty as a portrait-type lens. One school of thought says that you don't need a good prime (non-zoom) lens if you have that focal length in a decent macro*. The Canon EF-S 60mm f/2.8 macro that I use on dry land would be fine for portraits and also gets very good shots at infinity.
The real issue will be that macro lenses have narrow angles of focus: the one I use is about 25o, a friend's 100mm is 24o, and I've rented a 180mm (for insect shots; tripod and focusing rail required) with less than 14o. As a reference point, my 50mm prime is about 46o and the kit lens that comes with the cropped sensor Canons (the Rebels and 10D/20D/30D/40D/50D) and Nikons (the D40/D40x/D60/D90/etc.) are in the 74o to 78o range.
So a decent macro lens should focus at infinity just fine, but you aren't going to get that same wide scene that you are "seeing with your eye" when using that lens. Whatever width you do get is strictly due to lack of zoom/focal length. In that case, even the kit lens might be preferable, assuming a small aperture (i.e. larger f/stop number) and an appropriate shutter speed relative to available light.
HTH
John_B
* I disagree, only because I use a 50mm f/1.4 prime in low light situations or where the two stops make all the difference in the depth of field/bokeh (artistic blur) for the shots I'm trying to get. If you don't shoot those kind of shots, then the prime lens could very well be redundant.