Do Coerco poly water tanks split at the seams?

Coerco Poly Water tanks don't have seams so the short answer is no they don't.

There are no seams in Coerco poly tanks. Parting lines may be visible on the outside where the lid meets the wall of the tank that look like a join. This is where the lid and base moulds connect. However, Coerco tanks are not made from two sides joined together, they are formed as one solid piece during the rotomoulding process. Under no circumstances are the tank lids removed for transport.