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The third try was charmed.     Essential tools were a borescope, very bright flourescent worklight, and a 1/4" ratchet wrench, with Phillips bit and 14" long (in total) extension.  I taped the extension together so that it would not come apart in the bilge.

My heater had three screws in the hidden (outboard) flange.  The hardest part was engaging the Phillips bit into the screw heads, maneuvering the wrench by the handle end, 14" away from the working end, looking into the borescope, which screen seemed to show a reversed image.

I plan to Dremel a slot (or two) into the flange of the new heater, so that it can slide under the screw(s) on the outboard side, and so the screw(s) will not have to be removed in the future.

My original 1988 vintage Atwood EHM6-SM water heater failed recently, so I ordered a replacement.  In two tries, so far I have been unable to remove the old heater.  The unit has two mounting flanges on the bottom, fastened with wood screws to the plywood platform.  The inboard flange, wiring and hose connections are all accessible through the access door on the inboard side of the galley console.  The outboard flange is behind the heater and underneath the sink, hoses and wiring, and cannot be easily reached.  In fact, it is not even possible to see the flange directly.  I employed a borescope and determined that there are two Phillips mounting screws in place. 

Removal requires some sort of tool with a Phillips bit, approximately 14" long.  It is necessary to apply downward force on the bit to prevent it from "camming out" of the screw head, while also turning the screw.  I made up such a tool, but there is so little head clearance under the sink, so far I have been unable to get enough down force on the bit to make any progress.

Everybody replacing a water heater would have faced this problem.  Is there a technique that does not involve removing the sink or the drawer frame under the sink or installing an access opening behind the sink?
 

I installed an Isotherm SP2553 seawater-cooled unit in 2012, and have been quite pleased with it.  The main disadvantage is that it requires a special new thru-hull fitting, to replace the galley sink drain fitting.  The condenser heat exchanger is an integral part of the thru-hull.  Compressor noise is hardly detectable.  The refrigerator stays cold and most of the compressor heat dissipates in the water outside the boat, keeping the cabin cool(er).