GFCI Electrical outlet near outdoor fountain wall - Shutoff switch needed?

Electrical drownings are deadly serious

If you understood how gruesome they are, you would earnestly want to do everything possible to avoid them. Imagine someone drowning, and a rescuer jumps in to help them and starts drowning too, and another rescuer does the same... that happens. It happens A LOT.

As in the linked case, it's not enough to GFCI-protect the device alone, because water could get behind the GFCI device and contact unprotected AC mains. So it's also important to protect the wiring that even approaches the fountain.

Also in that linked case, the critical 10 minutes needed to get the girls on CPR and save them from brain death, was spent frantically searching for a way to disconnect power. Hence...

GFCIs can protect downlines.

A lot of people look at GFCIs and think "That's a receptacle. It protects things plugged into it. It doesn't protect anything else". That's true if you wire it that way... but it's also possible to configure GFCIs so they protect onward wiring runs. In fact every GFCI device has terminals specifically for that purpose: the LOAD terminals.

So you can use a GFCI some distance away to protect both the wiring to the fountain, and the switch, and the receptacle all at once. This can be a GFCI receptacle located somewhere earlier in the circuit, or a GFCI breaker in the panel. It's also wise to locate GFCI devices indoors - they last a lot longer.

There's one more thing. Some GFCI devices do not like being downline of a switch, and will often trip when shut off or turned on. All the more reason to site the GFCI before the switch.