Where would the best beaches be to sit on the beach and relax be? Ambergris? Palencia? Any suggestions? Will be there in beginning of July?

Placencia has the better beaches in Belize.

If you are looking for beaches Belize is not the place to go. Agree that Placencia is "better", but sorry to say think you will be disappointed. Went to South Water Caye for a few days and the little beach there was idyllic. That is the best beach I saw in Belize.

You are right, SW Caye is in a class of its own. I spent last weekend there, just wonderful.

Where would you recommend staying in South Water Caye? Should I stay there over Placencia? Also how can I get back to Guatemala from South Water Caye or Placencia to catch a 12:35 PM flight?

Two very different destiantions: SWC is 10 miles from Dangirga on the reef, about 14 acres, 3 small inns. Placencia is a long peninsual with a quaint village and a much larger assortment of restaurants, bars, hotels, inns and condos 19 miles from the reef.
You would fly from either Dangriga or Placencia to Belize Intl to catch your flight to GUA.