I definitely think you should add a timer, especially on one of the specific levels, where the mission briefing said, "Get to the docks, quickly!" It would add more excitement to the game, more importance, and overall make it more fun.

The timer could be like, 7 minutes or so, and if you don't finish the level before the timer runs out, you automatically fail the whole level and have to start over from the beginning. (of that level.)

Just an idea. ;)

Agreed. The mission briefing in several levels implies a sense of urgency, but you can take as long as you want to finish the levels with no repurcussions whatsoever.