1 - Cremation - I would leave the place blank and put "Cremated" in the description field of the burial fact. You could create a sepparate fact called "Cremation", but I don't. If you know where the ashes were scattered you could add that to the description field of the burial fact. If I knew the facility where the cremation took place, I would put that in a place that would not be part of my genealogy report - either the Fact Note marked private or the Source Text with the box to print unchecked or Research Notes.
2 - I generally don't record stuff that is proven not to be true; so I would "dump" the "as of" death date in the service record in a spot where it won't be printed - either a Fact Note marked private, or a Citation Text marked "not to print", or even "Research Notes" marked private so as not to print. That would be for "my eyes only".
3 - I would record the place "at sea" as close you know. If it was in Coral Sea, I would say "Coral Sea". Not all places have to be resolved.