The Ritual is not performed at any old cliff with water or any old Waterfall, it is done at Warrior Falls.
According to Wikia
The Warrior Falls are a set of waterfalls where the King of Wakanda and the heir to the Black Panther is coronated
The ritual combat performed is not just taken place in water, at the edge of a waterfall it's taken place at this specific location in Wakanda.
During the movie we can see that the throne of Wakanda and the mantle of the Black Panther goes to the best warrior and due to tradition each tribe had an option to challenge the heir/king for the throne.
I believe as time went on the other tribes did not bother to actually fight the king because they are happy with the way the king is ruling (T'Chaka and then T'Challa for a little while). Only the Jabari stick to traditions and want to actually challenge T'Challa for the throne.
However it is likely that traditions state that the ritual combat has to take place in a certain part of Wakanda in this case Warrior Falls.
You most also remember that the warrior falls is a place of coronation as well. T'Challa was about to be crowned King and the new Black Panther (before the Jabari interrupted) and a coronation cannot happen just anywhere, it has to be somewhere sacred or important in the comics
Wakanda's most famous water falls. Site of battles between T'Challa and Killmonger. It is also where Killmonger was killed, T'Challa was defeated. According to T'Challa the falls are a work of wonder even though he has been all across the world and into other dimensions. The cascading waters of Warrior falls still inspire awe.1
Yes the people of Wakanda do have a more broad, special connection with the "idea/spirit" of water. Especially the River tribe.
According to Syfy
In the movie, the River Tribe has dominion over the water ways. During the Warrior Falls scene, we see Nakiya dancing and her father praising what might possibly be a water deity. Since the Panther’s ritual battle takes place on the water it would be in line with most African traditions to bless the water before one enters it.
Most of the other tribes seem to have deities of their own. For example, the Jabari tribe seem to worship a gorilla or ape of some sort. Therefore it wouldn't be surprising if the river tribe worshiped some kind of water deity.
Also in many real life African tribes water is used to symbolize washing away of sin and as a rite of passage.
So in Black Panther, the blessing of the water could be seen as a way of purifying the combatants to make them worthy of the mantle of the Black Panther.