Well I'm going to go against the grain here and say that IMO she wasn't in the water all that time.
Looking at the chart for her disappearance - kindly reposted at 8.19 by ElenaJ - we have a Pisces Ascendant and therefore a Virgo Desc.
I was taught there are two ways of interpreting a chart of a missing person;
If you know the person use the 7th house ruler as significator - in this case Mercury
If you don't know them use the 1st house ruler as significator - in this case Neptune (or Jupiter using old rulerships)
All these planets are located in the Eastern side of the chart so, at the time she went missing, IMO, she was East/South East from where she was last seen.
However, she was found due West of the bench where the Police thought she fell in.
So something is wrong somewhere.
PussInBoots, because this is basically a learning forum, I thought it might be interesting to evaluate what happens to our interpretation if we follow your suggestion of using 7th for the missing person.
In her book Simplified Horary Astrology by Ivy Goldstein-Jacobson, on questions regarding the 1st house she in fact analyses a chart for Where is the missing Girl?, by assigning her to the 7th house, and turning the chart.
She refers to the missing person as "the undesignated girl".
She writes "When the status of the person is not clear, except that he is a missing person or a fugitive from justice, he is ruled by the 7th house. "
In the case of the chart she examines, the missing person was a student from India, who it was later discover, had committed suicide.
So, looking at the event chart for the time of disappearance of Nicola, the 7th house is in virgo, ruled by mercury.
Turning the chart, we now have asteroid Nicky in the 1st house.
Mercury, her significator is in the turned chart's 5th house. (11th of the chart), the house of love and creativity.
Granted we are looking at event charts (for the time of disappearance and discovery), not horary, but I think we can still safely apply the same rules.
Mercury, Nicola, is in Capricorn, a solid earthy sign, so we have now eliminated the water element. She would be in a dry, earthy place. A field?
Mercury exactly squares Chiron, showing her state of mental suffering, and moon is in the turned 7th, so there is a potential tie in with her partner.
Again, Goldstein-Jacobson says if the ruler of the missing person is in a succedent house (in this case the 5th), the person is nearby and easy to reach.
Moon in Aries last passed over Jupiter, which shows the past events relating to the missing person.
Jupiter is in the turned 7th house, and rules the turned 3rd and 4th. Jupiter relates to thoughts and travel.
Mercury, her significator, separates from a square to Jupiter.
Moon also separates from a square to mercury.
Mercury's next aspect is a trine to Uranus, of sudden unexpected events, near the cusp of the turned 8th house.
So this does point to the missing person's death.
The turned 8th is ruled by Venus (why does she keep coming to the forefront?), approaching a square to mars in the turned 9th and deposed by mercury.
Where is mercury? Should we use the directions of the turned chart, or the radical chart?
In her delineation, Goldstein-Jacobson uses the turned chart to find the direction.
In the turned chart, mercury is north west, which corresponds to the actual discovery of the body.
The Part of Death in the turned chart is at 4 libra, exactly opposing Jupiter, the last planet that moon passed over.
So, in reality, by turning the chart we do come up with similar results, except for the important point of water or no water.
We can only wait for the autopsy results, which should tell how long the body was actually in water, if it was for the entire time, or just recently.