I'm trying to figure out how the water weird from a weird tank emerges from the tank and how it should be represented on a standard grid.
The description of the weird tank magic item (Princes of the Apocalypse, page 223) says:
A water weird is contained within the tank. While wearing the tank, you can use an action to open it, allowing the water weird to emerge. The water weird acts immediately after you in the initiative order, and it is bound to the tank.
This part is pretty clear to me: the weird remains inside the tank until summoned, and the source of water for the weird is the water within the tank.
Therefore, the weird cannot leave the vicinity of the tank, per the water weird's Water Bound trait (Monster Manual, page 299):
Water Bound. The water weird dies if it leaves the water to which it is bound or if that water is destroyed.
All fine so far. To me, this means that the water weird must occupy the same space as the tank. Since the water weird cannot fly, this means that the owner must remove the tank and place it down on a space separate from themselves (to avoid drowning in it) and the weird would then occupy a 2x2 square with one square containing the tank. It shouldn't be able to move from this square unless another source of water is available.
Does that sound right?