Will we be able to take the amount of water that will be needed for this 18 month trip, or will they have to recycle the water that they take with them? I imagine that this will be quite a challenge because water is very heavy?
It's likely that a Mars mission will take a total of 32 months, not 18; the launch window for a fuel-efficient return journey requires a wait of over a year after arrival.
Humans need 2-4 liters of water per day. Assume a crew of 5; that works out to about 10-20 tons of water if you don't recycle it. That's a significant chunk of the overall mass requirement of the mission (NASA thinks the crew habitat and command module for a Mars mission will mass about 50 tons), so it's obviously attractive to consider a small water supply and a good recycling system.
However, one of the major concerns for a manned Mars mission is radiation exposure over the journey, and water makes a decent radiation shield. If you have to carry 20 tons of shielding anyway, you could do worse than carrying it in the form of water.