You can create a water plane by going to Plugins, SketchyPhysics, Buoyancy settings.
You can change the density for example if you put "500" in the box you can just about drive a car on it; put it to 0.1 it will sink pretty fast. You can also put a current on the WP by putting a number on the "current X,Y,Z" boxes.
Water Plane tips
If you are having trouble that when something goes flying all around or acts real stiff in the water the problem is that it is to small. You can fix this by putting the "worldscale" down, or simply scaling your piece a little larger.
Mixing Buoyant and Sinking Objects
Objects that float and sink can exist concurrently. An object's overall buoyancy can be changed by grouping some other objects inside it, where they will be hidden. Next set the Buoyancy Plane density to be the same as the SketchyPhysics Default Object Density, and run the simulation. The grouped object will float, while single objects will sink, depending on the size and number of objects nested into the groups.
In Sketchyphysics 2
It should be noted that a buoyant plane is not new to SP3. SP2 actually had a very similar, but very limited function. If the user typed in "makeWaterPlane" into the ruby console, a blue surface of "water" would appear and there would be a water like area below the origin.