I'm afraid I don't have personal experience with Marco rocks, but I should be able to answer a couple of your questions anyway. First, I'm pretty certain the rocks won't arrive in any significant amount of water. It's far more likely they'll be wrapped in saltwater soaked newspaper or something along those lines. A full container with water in addition to the rock would simply be too heavy to be practical to ship. As for pre-cured, you have to take that term with a grain of salt. Even if it is truly pre-cured, there will be at least a little bit of die-off during the shipping. Depending on the time in transition, how much organic material there was on the rock to begin with, how much rock you're adding, and how capable your system is to be able to handle a new potential ammonia source, you may be OK adding the rock directly to the tank, but there are a lot of what-ifs. If it is in fact pre-cured, I'd still recommend putting it in an isolated tank (or bucket) and monitor the water parameters for at least a few days. On the other hand, if you don't currently have any livestock, it would be perfectly fine to add the new rock to your tank right away. Just make sure you're not getting any detectable ammonia levels before you do add fish or invertebrates.
