Crete is the largest island in Greece and boasts some of the best beaches I've ever been to. I wasn't a beach person before visiting here, but I'm fully converted now. The combination of beautiful water for swimming and warm sun is just so relaxing, and it's a far cry from the insane humidity of the Southeastern US, which is where I've spent the most beach time.
Elafonissi
Elafonissi is famous for its pink sands. The sand is really only a faint pink color, but it's still a stunning location. It's located in a lagoon that leads to an island, and you can wade through the water across the beach to reach the island. It's a great beach for kids because of the huge stretches of shallow water.
Balos
Balos is on the northwestern tip of Crete. It's a little bit tougher to reach, as you have to either drive a dirt road and hike down, or take a boat that docks on the nearby rocks. If you take the boat, you can stop by an island with the ruins of Gramvousa Castle. Exploring ruins is one of my favorite things to do, so I definitely prefer the route by sea!
Preveli
Preveli, which is also called the palm tree beach, is on the southern side of Crete, in the middle of the island. You have to walk down quite a few rocky steps to get to it, but it's well worth it. There's a river that feeds into the sea from a gorge, resulting in a grove of palm trees next to the beach. Despite the long walk down, there is a small cafe where you can buy food to eat in the shade of the palm trees while enjoying a view of the ocean.
Marathi and Loutraki
Marathi is right next to Loutraki on the Akrotiri Peninsula, near Chania. Loutraki has a grassy area with beach chairs, which is perfect if you hate sand. There's a ladder that leads into the beautiful blue-green water, so swimming is still an option. Marathi has a larger beach area with several restaurants. My favorite part about Marathi is the island that you can swim to (I'm bad at judging distance, but it's probably about a quarter mile out). The water and swimming is absolutely fantastic at both!
Agia Roumeli
The Samaria Gorge hike starts in the mountains, where most people get dropped off by a tour bus, and continues through fantastic scenery to end at this little village by the ocean. It's only accessible by foot or ferry and while not really worth visiting for the beach itself, it's a fantastic end to a long day of hiking.
Stavros
Stavros is the beach featured in the classic movie "Zorba the Greek". It also has a small cove for swimming and multiple restaurants nearby. There's also a cave that you can hike up the cliff to explore, although it's quite a steep climb. We found a slightly quieter spot nearby that serves fantastic food right next to the ocean. Crete is full of little spots like this!
Falassarna
Falassarna is on the far western side of Crete. To the north of the beach are some ruins you can explore from an ancient city that once stood here. The contrast of the mountains with the ocean makes this beach especially beautiful. There are several beachfronts along this stretch of coastline, which makes finding a spot easier when the crowds set in.
Sweetwater Beach
This beach is only accessible by water taxi or by hiking the E4 trail, which adds to the remote feel. It's called Sweetwater because if you dig a bit in the sand, fresh water comes up. We stopped here on our kayak trip and ate at at the small taverna while enjoying the beautiful view of the beach. Note that this is also a nude beach. Some people joke that all of Greece is a nude beach, but the reality is that in some areas it's more acceptable than others.
Seitan Limania
My personal favorite! We refer to this as "Secret Beach" amongst ourselves, although it's not so secret anymore. It's a stunning, tiny beach in a small cove that requires a short hike to reach. The drive down has some harrowing switchbacks with fantastic views, so make sure you brush up on your manual driving skills. You can also climb on the surrounding rocks to do a bit of cliff diving!