Ever seen a JetSki, PWC, or Jet boat "take off"? No lag time that I know of.
Check out
http://www.omnithruster.com
Their thruster starts up and runs constantly (when you want it to be ready for use). It thrusts in both directions which cancels itself out, then it blocks water to a side causing an imbalance to push the boat. This way the thrust is up to full power at all times.
They also have nozzles you can direct 360 degrees. So you could drive the boat forward or backward as well as side to side. Could even drive the boat home in an emergency.
Another advantage to thrust from a water jet is that it works when you are underway. No cavitation such as a propeller thruster.
Mostly used on larger ships. Used also for holding position when working with underwater gear.
Very expensive.