Just to clarify: have the purple areas been removed/fixed? Besides not being able to build, these areas cause vehicles to 'ice-skate' while on land. Very annoying. Lost a Jagdtiger there once because it couldn't back up because of the skating.
SHORT ANSWER
Yes the water areas(purple) have been optimized.
LONG ANSWER (My weak understanding of it)
COH has a water object that is placed on a map. This will 'fill' the areas with water up to a height that you declare. In most games drawing water can be a very costly operation as you need to draw the entire world upside down to create the reflection on the water. It is usually rendered to a low resolution texture in memory and then a shader program is used to draw the blurred reflection with a modulated ripple effect. There are of course many other operations such as fogging,etc to give it a realistic look.
It has always been a good practice to not have too many water objects on a map. So on a map like this where you need a lot of disconnected water sources you need to have multiple water objects which could kill your FPS in game (does it really anymore?).
Or you can use one water object and have the separate water areas connected by a very low terrain area. The COH engine will continue water under terrain if its height is very close to the water level. So you can make bumps and rocky areas on your beachs, river banks, etc.
Here it is used to connect all of these small lakes. Originally the low terrain connections were very large and created issues where the game engine says "If its purple it is not land. It is water". So you had vehicles acting weird and you cant build objects on that territory (MGs, wire, sandbags, etc).
The large purple 'water' areas have been stripped down to small lines where possible.