In this stage Trip, Monkey and Pigsy are stealing a submarine. Trip and Pigsy are on the submarine as it is being moved to the dock and Monkey (of course ;) ) has to clear the way for the sub to move on it's track.
There are 2 time sensitive bits in the level, past the first airlock Monkey needs to shoot two obstacles and in the dock the sub starts to be lowered. In the dock area, the sub won't lower until Monkey gets to the last platform, so you have plenty of time to explore and get the orbs.
No boss in this level, just inconvenient mechs.