Here from settings:
Many different underground streams spring from the walls of the first and second levels, some of which were formed from time immemorial, and join together at the bottom of the stalactite, where they form a lake that submerges the two lower levels. Somehow the water gets out, since Yliakum hasn't been flooded once in recorded history. The flow of the streams varies as times goes by, and the level of the lake changes accordingly. Sometimes the lake level is so low that almost half of the seventh level is revealed, but other times the level is so high it almost reaches the edge of the sixth level. The springs here supply a constant source of drinking water for Yliakum.
The seventh and eight level, both usually submerged, host the Nolthrir, an Elven race who have been attending the seaweed for thousands of years. With the influence of time and the Crystal, the Nolthrir evolved into an amphibious species, which enabled them to remain under water for long periods of time. After centuries of crossbreeding, the seaweeds grown with such skill as the Nolthrir possess boast different qualities. Some of these varieties are used as food, while others are tanned and woven to make clothing. Still others have medicinal properties, but most of the seaweed is dried and used as fodder for Pterosaurs, Megaras, and cattle bred on the first level.
end.
from this explaination having the city at the upward limit of "high tide" on the 6th level seems resonable. The tides might not be daily but seasonal, and the city could simply be on a spring fed resevoir type lake.
for the sails, wind is kinda hard to get around even in here . . .
wind is caused by changes in air pressure
http://eo.ucar.edu/basics/wx_2_c.htmlthis is due to the weight of air itself
http://eo.ucar.edu/basics/wx_2.htmlif there is rain . . .there is atmosphere and likely, wind.
