Seriously, it's a joke. The line of sight of both ground units and ships is pathetic, like they're blind people in pitch darkness. My armies get massacred because my ranged units can't see who is shooting at them. Can anyone recommend me a mod that deals with the issue? I need the LOS for all units to be four times bigger than the current one. I'd use cheats, but they don't work in adventure mode.

You could try pairing artillery with scout units that have larger line of sight. I realise that's possibly a bit more tactical than you want but it's how these things are meant to work.