My problem is that I don't really understand which purpose the env_cubemap entity has. I never used it and didn' t have any problems with reflections or whatever the entity does.
Though one thing to note is that other players on your map DO NOT have a default cubemap, meaning that they will see a missing texture on any reflective surfaces. Keep this in mind if you start having a lot of reflecting things in your map.
Also yes, I have a surface of water in one of my maps, and although I have never had any problems with it, other people seem to have had problems with it (a few !fb give water a cubemap)I never used it and didn' t have any problems with reflections or whatever the entity does.
Note that based on the skybox it can give you the default one. That cubemap is simply the skybox. When not placing cubemaps it normaly will use this one, with custom sykbox textures it usualy does not. So if you use a default skybox not placing a cubemap is better than placing 1 at a random spot as the default skybox cubemap is often equaly good looking as a cubemap for a wrong section of the map.You may have trouble when packing custom assets in to your map, if you did not place any env_cubemap entities. For an alpha map, as Muddy says, it cuts down potential problems if you include at least one.
Creating default LDR cubemaps for env_cubemap using skybox materials:
skybox/sky_sandstorm_01*.vmt
! Run buildcubemaps in the engine to get the correct cube maps.
Creating default HDR cubemaps for env_cubemap using skybox materials:
skybox/sky_sandstorm_01*.vmt
! Run buildcubemaps in the engine to get the correct cube maps.
*** Error: Skybox vtf files for skybox/sky_tf2_04 weren't compiled with the same size texture and/or same flags!
Can't load skybox file skybox/sky_tf2_04 to build the default cubemap!
*** Error: Skybox vtf files for skybox/sky_tf2_04 weren't compiled with the same size texture and/or same flags!
Can't load skybox file skybox/sky_tf2_04 to build the default cubemap!