This was written for Submariners lost "on eternal patrol"; but it's the only appropriate memorial I know:
”There is a port of no return, where ships
may ride at anchor for a little space.
And then some starless night the cable slips,
leaving only an eddy at the mooring place.
Gulls veer no longer, Sailor rest your oar.
No tangled wreckage will be washed ashore.” 
