Given puking in a space suit or a hard hat diving can ruin your whole day as can having a warships crew down for the count can make you dead so there is a literature concerning this subject.
There was a seminal piece done by the US Navy focused on the operational forces (so it was written with words of few syllables *grin*) in the late 70s. It was a review and statistical analysis of a lot of previous work if I recall correctly (I no longer have a copy - too much housecleaning). It showed three principal factors: frequency, amplitude, and spectrum. People have some frequency of motion to which they are most sensitive. The degree of sensitivity also varies from person to person. The extent to which people are sensitive off their particular frequency also varies.
This is why there are people who don't seem prone to motion sickness at all, others who get car sick but not sea sick, and some who are very sensitive to motion of any kind.
Not in the study but just from my empirical experience hydration is indeed a very important factor.
I have also observed psychological factors e.g. being out of sight of land.
I have been sea sick twice in my life. I don't recommend it. Fortunately both were in fairly benign conditions (which goes to the cited frequency sensitivity parameter). I have also had the flu while underway which while unpleasant was not as bad as being sea sick.
I am not sure a roughly 2.5% in water density is negligible.
That depends on what you are considering. Certainly static stability is a factor (see http://media.web.britannica.com/eb-m...4-8CD56C34.gif
) and note you can load a ship more heavily in fresh water. I don't remember any concern about dynamics at Webb about fresh v. salt water.
On an empirical basis, running from Newport to Annapolis (salt to brackish to fresh to brackish) or Annapolis to Norfolk (brackish to salt) I can't say I've detected any change in dynamics.