During one of my Boss Fights one of my players was attempting to use Maelstrom on the boss on a map with various terrain differences and I am in need of Insight or help Involving how the spell interacts with the differences, Both for future information and current questions for the next session that takes place with the same boss. (I counter spelled the first one).

Specifically I am curious about Maelstrom being casted at the lowest height differential of an area and the highest height differential. 
as a example the map has a lower platform 5ft below the base level where as there is another platform 5ft above the base level with a large hole being on the opposite side of the higher platform. leaving a 10ft gap between the lowest and highest platforms with a massive drop on the far side of the highest platform.

at first I thought to rule it as the water not contained on an area would flow out of the spells control leaving the original area still under the effect but all others to be null and void. BUT the spell does not explain if the water is magically controlled in this 5ft disc as the water does not flow out normally if it were to extend out to its maximum of 30ft so i am torn between rulings and i am curious if the 5ft disc of water forms across height differentials flowing 5ft over any surface within its 30ft Radius......

please someone help me with this dilemma!!  

This is interesting, the way Maelstrom is written doesn't really describe how it handles changes in height. I would say it coats all terrain within 30ft of the targeted point in 5ft of water. I would let the spell jump up and down height changes as long as it is within 30ft of the targeted point. I would even allow it to effect squares on the other side of a pit trap regardless of if the squares inside the pit trap are within 30ft and also effected.

The best way a player can take advantage of the terrain is to cast it on the opposite side of a pit from an enemy and pull them into it. This doesn't seem broken to me to allow because there are a number of ways players can force enemies into a dangerous fall.

I wouldn't worry to much about how the water behaves if it leaves the spells area. The spell doesn't require a body of water so it presumably conjures the water. If you don't want to deal with water flooding outside the spells area I think it is fair to say it evaporates into mist, after all it is magic.

I would assume that it's intended to describe a 30 foot radius, 5 foot deep cylinder of water; the part that makes it tricky for me is "The point must be on the ground or in a body of water", which suggests that it's supposed to be created at a lower level rather than to create a disc of water suspended in the air.

Overall it's a poorly described spell; personally I'd probably rule that it's intended to be placed on level areas, so putting it on a small raised platform would have the effect only apply to the platform itself with the excess water pouring off the edges, but I'd also ask what the player's intent is behind the spell as there may be some compromise, such as allowing a one-off Strength check to pull the enemy as the maelstrom initially forms then spills over the edges?