Can you land IFR at sea?
Well there is at least one case where this is true and it's used by the military - A Case 3 instrument approach to an aircraft carrier.


                
            

The approach is essentially equivalent to a terrestrial military TACAN approach coupled with a precision microwave landing system on final approach.  See the basic plate below.  Both IMC and night VFR conditions require a Case 3 approach per NATOPS.



As to civilian seaplane bases, the answer is actually, yes, some do.  One example is the GPS RNAV approach into Greenville Seaport (52B) in Greenville, Maine.



As for an instrument approach to land a seaplane at a arbitrary point in the middle of the ocean, no, I am not aware of anything like that.  Maybe one day an advanced version of GPS or some similar satellite based navigation system coupled with an advanced INS could allow a pilot to arbitrarily pick a point then create a custom approach which provided lateral and vertical guidance to the landing site.  Even if you could do this, you still face unknown hazards like surface winds and waves (seaplanes were never made to brave rough open ocean swells).  It would be virtually impossible to safely land on a moonless night.