in response to the water comment i think that perhaps the blue you have chosen is a little over strong and although water is though of as blue thats because of the atmosphericl lighting it gets in daytime. At night your water changes colour. I think this might be one of the reasons i find it harder to accept the time of day as night
A little about water
At night there is very little light wich tendas to make water apear more reflective as there is less light entering and being reflected from inside the volume witch creates the transparent effect. At night much more lighe is shon from the surface so dont go for high transparency. If you want night look then from outside i would perhaps even surgest zero transparency
from inside have some transapency but not much since less light is penetrating the surface. I dont know if you have fog volume inside your water or not but the most unreasislitc thing about water in ET is how clear it is but like you say fps is king and this as quite expensive
On thing i foten though about with night time water was the posibility of making an under side for the skybox that mirros the top then using the skybox shader as the water then adding another pass to blend a semi trans blue or silvery texture over the top. This would go somewhat to faking a reflection without killing your fps. reflective water done make water MUCH MUCH more realssit as in reality the actul colour of water is hard to see since you mainly see the relfected colour on the point you are looking
of course you would have to make this ony show on the top side. There is a shader command that will to place the sahder on the top of the water then it automatical palce a differend shader on the underside of the texture rather then using the common both sides technique for water
EDIT// oh btw i said above that i am very busy well luckly i got a lot of work out the way finaly have a bit more free time sadly not enough to return to full mapping but if your looking for some help with terrain/shaders/lighting i would be willing to lend a hand as i can see this map clearly has some great potential. drop me a PM if your interested