Always in search of unique underwater photography methods (2015 marks my 40th year taking underwater photos, so I need some stimulation). A few months ago, I began experimenting with adding extension tubes on my Olympus PEN camera's.  I already owned two prime macro lenses for the PEN camera's:

• Olympus M.Zuiko Digital ED 60mm F2.8 Macro 
• Panasonic Leica DG Macro-Elmarit 45mm F2.8 ASPH OIS
  

So with the addition of a set of inexpensive extension tubes I experimented with what could be achieved without any great outlay and achieve more than 1:1 macro underwater photography AND produce high quality images.
Extension tubes are just that, a tube that connects to the camera body and then the lens attaches to the extension tube. There is no glass to add distortion, all the extension tube does is move the lens further from the camera's sensor (which without getting to technical, adds additional magnification and closer focusing ability to the camera's prime lens).
The extension tubes I used were very cheap, from memory around USD$30 for a set of two (see below for details). The important part is that these have electronic contacts, so the camera can talk to the lens. Some very cheap extension tubes available for the Micro Four Thirds Mounts do not have this. There are also more expensive extension tubes available, that look to be made from heavier material, but offer no photographic advantage.
I tried various combinations of the prime lenses. One of each extension tube and even with two extension tubes stacked to get more magnification. The results were impressive with some combinations easier to use than others, but all combinations presented with photographically good results.
I needed an extension ring for the housing port for some combinations, others worked with the standard port. See the chart below for more details. Click on the photos below to see the results: