"The king probably died around 1202 BC, but his mummy was not found within his tomb. In the 19th century, this apparently added to the speculation about him being the Pharaoh of the Exodus, since that king's body would have probably been washed away in the Red Sea. However, that theory was confounded when, in 1898, his mummy was discovered among 18 others in the mummy cache discovered in the tomb of Amenhotep II"
-- http://www.touregypt.net/featurestories/merenptah.htm
"In the skull of the mummy of Merneptah is a round hole made by a sharp, pointed instrument. To explain this injury it was thought that a surgical operation had been performed on Merneptah's head during his lifetime, or that it was made after his death. But this hole is apparently the result of the mortal wound at the hands of his assassins."
[G. Elliot Smith, `The Royal Mummies', Catalogue general des Antiquites Egyptiennes du Musee de Caire (Cairo, 1912), p. 68.]
[James Harris and Kent Weeks, `X-raying the Pharaohs', New York, 1973, p.157.]
-- http://www.specialtyinterests.net/israel.html
"He died a natural death in high age."
-- http://www.neareasternarchaeology.com/encyclopedia/index.php/Merneptah
-- http://www.baghdadmuseum.org/ref/index.php?title=Merneptah
-- http://merneptah.biography.ms/
-- http://lexicorient.com/e.o/merneptah.htm
I exhausted every search engine and biography of Mernetaph and that's all information it had concerning his death, nowhere even speculating a drowning... but rather natural death or a assassination.
the sad thing about all this information is that it's worthless, because all of them are contradicting each other. Making it impossible to know who is speaking with knowledge and who isn't, the only thing they all seem to agree on is that he's dead. Then for that reason it is fairly easy to handpick your (good or bad) source information and put it in your own context like Muir did... similar to Michael Moore methods who are also easily believed if you don't check his sources or disregard contradicting theories. (neither am i impressed by your references to 'elite' archeologists... archeologists tend to disagree with one another as well... just like Dr Khaliffa isn't seen as a historical authority by the majority either)
Yet overall, nowhere from non-muslim sources (or should i say a non-muir source) is indicated that he drowned... It adds to the assumption that muir is reaching and twisting and that muslims are a gullible lot who don't double check sources.
Yet i advise to read Answering Islam about the contradiction... who has far better research then me, and more sources at hand
http://answering-islam.org.uk/Quran/Contra/qi025.html