Ice water does two things. The cold temp helps harden the fat, which helps make minimal fat work harder when you do go to cook. The water makes it seem juicier than it would otherwise be. It's mostly a trick used for "spa-burgers" to make too-lean meat moist. At the end you're diluting your hamburger with water, and that's not a good thing. It should be unnecessary if you use good, fresh 80/20 -- something most people don't -- don't mishandle it -- something most people do -- and don't overcook it -- something most people also do.
Sometimes I add a little crushed ice to the well-chilled meat I'm grinding when it's still in the tray, to help the fat keep its integrity. Paranthetically, I almost always use crushed ice when mixing sausage.
If you must add extra moisture, reduce beef stock with red wine and Worcestershire, chill, and use instead of water. Or try a little chilled Cognac or dry Madeira -- you'd be surprised. Generally I don't add anything to the meat before forming patties because it means mixing going beyond blending, and even minimal mixing is too much. Waaaaaaaaaay too much.
Even though grass fed meat is leaner than grain-fed, you should still be able to approach a ratio in the ideal range of 85/15 to 75/25. For ordinary, good quality, Choice or Angus beef I prefer 80/20. Some cuts, like tenderloin are so inherently rich and lean, that lean suits them. Others have so much marbling they're not only difficult to lean, you associate their taste with plenty of fat, and their fat is especially good, like rib-eye. Wagyu does pretty good with lots of fat, also.
Of course those special cuts and breeds are too expensive for most operations and purposes. On the other end of the credit-card, if I'm grinding Select beef, I like to up the fat ratio as well.
Traditionally, "ground sirloin" is sold lean to very lean 85/15 - 90/10, but even though it sounds great it doesn't make great burgers in my opinion. I'd rather use chuck. I think I said earlier that my ideal mix is brisket, chuck and bottom round -- which is not an uncommon custom blend. Since I'm not cooking professionally, consistency isn't a big issue. So if tri-tip is on sale, I might go with that alone. Think of tri-tip as ground sirloin which tastes as luxurious as it sounds. Anyway, cuts and quality make big differences.
Grass-fed. I don't know from grass-fed. But I think you should grind a good mix of cuts in several batches -- each with a different amount of fat, and see what you think. It's going to take a phone call or two to your butcher to get the right stuff in the right amounts, and maybe an extra call or two if you want him to grind.
If you do have your butcher grind -- remember to specify that you want the meat as loose and non-compacted as you can get it. I think most commercial operations are actually better off if they have a butcher do it for a lot of reasons; but it has to be done no less than three times a week -- ground meat doesn't stay fresh for more than 48 hours. 24 if you're ultra quality oriented.
Ed and everyone else who commented on the evils of over-compacting and over-mixing is spot on. Ground meat will be "fluffy" straight from the grinder. If you store it in deep trays it over-compacts. If you over-mix, it will over-compact. If you form the patties too vigorously, the meat will over-compact. You want the patties just sufficiently formed to hold together. Hamburgers made from over-compacted meat are chewy. Burgers made from over-mixed or over-handled meat are greasy -- even with very little fat. Nothing quite like a chewy, greasy burger. Fresh ground, medium-rare and easy does it.
Hope this helps,