How about option 3, the way it's done in the real world: a solenoid valve?
Google "sprinkler valve" or "solenoid valve", or just walk down the plumbing aisle of your local borg and look for the sprinkler valves.
I'd probably look for a model that will operate on 12VDC, since that's pretty easy to come by and it's a well-documented arduino-level switching problem.
When figuring out your arduino circuit, treat it like a big relay, only for water.
-j