You can just use a differnt cubemap depending on what "time" it is.. this is easy enough. Make a function with a while loop like so:

function change_cubemap()
{
while(1)
{

if (timeofday==noon)
{
if (mat_water.skin1<>noon_cube)
{
mat_water.skin1=noon_cube;
}
}

//and so on
wait(10);
}

}