Do I need a grounded conductor (neutral) in my subpanel if all my loads are all 220v (i.e. 2-phase)?
It sounds like your pond electrical subpanel is illegally wired. Your electrical system is a “grounded electrical system”, therefore your electrical subpanels MUST have a neutral at a minimum, even if it is not being used (in your case it IS being used by the GFCI). You may NOT run an “ungrounded” electrical circuit within a “grounded” electrical system. The “Grounded Conductor”, also called the “Neutral”, must be supplied to each circuit within a grounded system per NEC 210.4, NEC 250.4 and NEC 300.13(B), even if it is not going to be used by the load.
Your subpanel must also have an equipment grounding conductor (EGC) sized per NEC Table 250.122 as well (see NEC 250.32 for more information). NEC Article 680 actually requires additional grounding in the form of an equipotential grounding system (plus the regular EGC), which must be tied back to your main electrical panel. Failure to install a neutral and an equipment grounding conductor is a very dangerous situation.
The Engineering Experts at E&S Grounding Solutions