One
reason straight DR in zero visibility may be frowned on with planes.

no, the reason it is illegal is that you can not know where you are. period,
as in can not.

DR in a boat is okay when you are hundreds of miles from any rocks. It is not
okay when there is anything around to hit.

recreational boats regularly hit the rocks in fog upto recent history with the
advent of cheap Lorans and later GPS's and to a limited extent RDF (which had
much higher degrees of uncertainty).