Why is my outdoor GFCI outlet failing so often?
Based on your description, it sounds like the GFCI is with an outdoor receptacle. That makes it susceptible to weather, and in this case also to spray from the waterfall.

As noted by Ed Beal, a high-quality Weather Resistant GFCI receptacle should do just fine. If you are replacing each year with an ordinary GFCI receptacle then an upgrade to WR should provide much longer life. The extra $10 for a top-quality WR instead of a regular non-WR really isn't very much. Add a few $ for an in-use cover and it gets even better.

But if you already have WR and it hasn't solved the problem, or if you want a little more peace of mind, switch the receptacle for an ordinary weather resistant receptacle. Install GFCI either at an upstream receptacle or in the breaker panel. That will provide the same protection but move the electronics to a safe dry location. Everything else (except the pump of course) is passive and not as affected by water unless there is an actual fault - which the GFCI protects.