You can get through a Guatapé day trip on English and hand gestures, no problem. But if you actually want a local to relax around you — the boatman, the woman selling obleas, the guy running the billiards hall downtown — a few real phrases go a long way further than "hola" and a smile.
The basics beyond textbook Spanish
This is paisa territory (Antioquia's regional dialect), so you'll hear "¿Qué más, pues?" as a casual "what's up," "parce" or "parcero" for "friend/dude" constantly, and "¡Qué bacano!" ("how cool") dropped into conversation about pretty much anything — the view from La Piedra, your Spanish attempt, the weather.
"Guatapense," not "Guatapeño"
People from Guatapé call themselves guatapenses — you'll see it on festival banners and hear it in town announcements, especially during the big annual festival. Using it correctly (instead of guessing "guatapeño") is a small thing that locals notice.
Around the reservoir specifically
Locals call the reservoir "el embalse" far more often than "el lago" (lake) — technically accurate, since it's an artificial reservoir, not a natural lake. You'll also hear boats referred to as "lanchas," and a private boat trip as "una lancha privada" versus the shared "chalupa" party boats.
Ordering like you've been here before
Ask for "una fría" and you'll get a cold beer without further explanation needed. "¿Cuánto vale?" ("how much does it cost") works better in casual spots than the more textbook "¿cuánto cuesta?" — both are understood, but the first sounds more natural here.