Out back or is this why you cleaned the garage? I would be concerned about a pond liner. Knowing you, there is no way this is just for her sharks. You'll be shooting for a million paly's. All the what ifs with a liner - UV exposure, effects of salt, rock slide, tree roots/branches, weed wackers... Repair could be a daunting task and you'd be stuck with what to do with a 3 foot Acan while the hole gets patched!
If you build indoor, that eliminates most of the above, but still need to know liner won't get brittle in time. Doubling up seems wise.