Ice harvesting can be profitable and useful under a certain number of circumstances. Roughly, ice sells for about 100k ISK per unit. What you'll want to do is hook up Ice Harvesters to your barge, in addition to Ice Harvesting Upgrades (low slots). There is a belt in Agal that contains ice, and can be mined by anyone that has the Ice Harvesting skill. When you warp to the belt, you'll want to bookmark a piece of ice near the middle of the belt and warp to it. This way you wont be mining at the entrance point to the belt, which is where you might find other people mining as well. After warping to your bookmarked spot, target the ice and activate the harvesters. After anywhere between 4 minutes and 7 minutes, you'll get 1 piece of ice for every miner you have activated. This ice should then be jet-canned (or into secure containers) where a dedicated hauler will pick it up and haul it back to the station.
A man reason for mining ice would to be able to refine it into fuel for a POS (Player Owned Station). Refining different ice yields different fuels for different POS. Amarr ice (Agal) is good for Amarr Control Towers (POS). Like wise with Gallente ice and Gallente Control Towers. Unless you have a POS, there is not much reason to be mining ice.
However, I mine ice when I am AFK or doing something else in the background for 1 reason: very little work is required. Being in a hulk, I have 8,000m3 of space, and ice takes up 1,000m3 of space per unit. With my 3 Ice Harvesters, i can wait a full 3 cycles (I drop 1 into a secure container) while doing whatever it is I am doing. I then return after the 3 cycles, dock, unload cargo, undock and continue harvesting the ice. The 2nd go around is the same as the first, I'll drop off 1 ice into the secure container in order to make sure my hold is not overfilled. On the 3rd "session" of harvesting, I'll pick up the 2 ice in the container, and only mine 2 cycles (6 ice from the 2 cycles + 2 ice in the container = 8 total ice = 8,000m3). That will fill me up, and I wont have to continually put ice into the secure container. This is a great method (you can adapt it using any barge or exhumer as you need) to make ISK while being partially AFK. The last time I harvested ice was when I had an English paper due the next day, so I stayed up all night writing it and mining ice. I only had to check up on EVE every 12-14 minutes or so, so I was not terribly distracted from my paper. By downtime (when I had finished my paper too), I had harvested 1,600 ice with very little work. Using the figures above, that turns out to 160,000,000 ISK worth of ice. However, I started selling mine at around 150,000 ISK and now at 125,000 ISK. Which means that that 1 night of writing my paper and AFK-playing EVE, I made over 200 MILLION ISK.
Now, while these figures seem large and would likely convince anyone to start mining ice, you've got another thought coming at you! Ice harvesting is NOT as profitable as regular ore mining!!! The only advantage it has is that you do not have to do as much work, but at a significantly reduced income. My suggestion is to continue mining ore unless the following hold true:
1) You are trying to supply fuel for a POS
2) You are doing something else, and can only check your computer once every 10 minutes or so, yet still want to make ISK.
NOTE: I use a Hulk with a faction shield booster! The Hulk and ANY exhumer can tank MUCH better than any barge! If you are not in an exhumer, you WILL NEED protection against the occasional rats that will show up. However, if you can perma-run a shield booster and are sitting in an exhumer, then you can go completely AFK, and not have to worry bout the rats. If not, I suggest getting combat protection from a corp mate or someone else. I've seen Retrievers die in fields plenty of time, due to lack of combat support. However, ice field rats are the same as asteroid field rats, so whatever protection you use while mining, should also be used when ice mining.
If you have any other questions, let me know.