It’s off the beaten path and you may need some flexibility in your schedule. We took a backpacker bus from Antigua (you could take one from Guatemala City) to Semuc Champey, then another bus to Livingston on the east coast. We took a large local bus from Livingston to Chahal, then got picked up in a pickup truck to get to Oasis Chiyu, which may be the only place to stay near the falls (if still open). Other options are ~30 miles away.

If you don’t want to worry about logistics then a tour might be a good option. You can take a tourist / backpacker bus to Semuc Champey from Antigua or Guate city. You could also hire a private van with driver if you have a small group or family. Renting a car is a possibility, but I wouldn’t recommend driving in Guatemala. The transportation can take ~8+ hours. Las Conchas is another ~6ish hours from Semuc Champey.

Finca El Pilar is a farm with natural pools just outside Antigua. The closest beach west of Antigua is Monterrico, but not very swimmable due to the currents. There’s also Sipacate, Las Lisas, and Tulate. I wouldn’t necessarily visit Antigua, Guatemala for the beaches, but there are some if you are dead set on visiting them.