I'm surprised it can't be damaged by water, that requires investigating. It should be damaged by water on all maps that other players can be damaged by water. If anyone can replicate that, it's bug report time.
I'm not sure about after the confusion of the 1.13 update and all the problems that brought, but on 1.12 in my view Halberd was pretty balanced. I'd need to play it and fight it more to work out if it has changed at all since then.