THE simple business of getting to the other side using the Cross-Harbour Tunnel at any hour of the night or day is becoming a real hassle. All forms of obstacles - from cordoned off lanes to police checkpoints (not to mention the endless ''maintenance'' the tunnel seems to undergo these days, adding to the misery) - stand in the way of a smooth, stress-free journey. The days when it was possible to drive up to the entrance without facing terminal gridlock and make one's orderly way through the tunnel always being able to use one of two lanes are but a distant memory. Perhaps the authorities might consider cutting off the tunnel completely to motorised traffic and flooding it to half its height. That way we could go back to using sampans to get across. It could not be any worse than it is now.