Good point, Sir William, however ...
I realize that "died at sea" is a commonly seen phrase, but that could be solved by entering the death place more specifically, i.e., "transit between A and B", "the middle of a cruise", "the X ocean" (if drowned), etc. Perhaps the died at sea cases might sound awkward, but the number of people this would affect is miniscule by comparison to those who died on land (note the use of yet another word to link death with location).
If we really want to allow for all the different possible words, there could be an additional field for selection of "in, at, whatever" next to each location, but my personal opinion is that it would be much more trouble to do than it is worth.