Our cruise is in Belize City for a day. We are just looking for a quiet day on a beach or resort by the beach. Clean beach and water, maybe water activities but not a must have, beach bar or restaurant, maybe some music. Any suggestions. OR if you have any suggestions on things to do but we have a rather large group....20

Stay on the boat

I can't help you with Belize, but I do hope you will ignore TravelHat's recommendation. The people of most cruise ship ports depend on visitors and their money. Don't be an ugly cruiser, get out and visit with the folks!

When you get to Belize City you will be tendered into the shore. The area has a fence around shops in a two block radius. Tee shirts and over priced jewelry that you see everywhere else.
We attempted to go out of the fenced area and were swamped by guides that wanted us to rent them for the day. Very aggressive. We walked to the left seeking out the city center to see some culture and buildings. We have been to many islands in the Caribbean but when we crossed the bridge into the city we didn't make it one block.
We returned to the "safe" area and went to the one restaurant that you could get a drink and lunch. Drinks were terrible and after an hour and fifteen minutes lunch showed up wrong, cold and tasteless. We paid a bill and headed back to the ship. I guess one one hand we saw it. The boat ride back out to the ship is pretty long as well.
You may need to do some research on an excursion away from the city but I would check it out. We were pretty disappointed because we have always seen great portraits of Belize on the travel shows and hear about all the expats retiring there but rest assure it is not in Belize City or close enough for a day trip.

Sorry Nutmeg but I have to agree with TravelHat - stay on the boat.

The best thing about Belize City was seeing it fade in the distance as our ship sailed away. We've cruised all over the world and Belize City was, without a doubt, the worst port of call we've ever made. the area around the tender dock was just as bad as TravelHat described. We took a land tour up New River to the Mayan ruins of Lamanai which took as twice as long to get there then it did for us to explore the "just OK" ruins. The whole day was a waste of time and money.

Stay on board the ship.

Thanks Fletch, I knew I wasn't alone.

Been to Belize twice, inland and the cayes. The country is stunningly beautiful but Belize City, not so much. And the good stuff is quite far from the port. If you're on a private tour and miss the sailing time, well....(shudder)

Enjoy a quiet, uncrowded day on-board the ship.

While on the ship, plan a trip to come back for a week or 10 days and do it right.

Not really sure how many days or ports are included in the OP's cruise. We were on a quickie over Christmas and the two stops were Belize and Cozemel. We felt compelled to get off. After we just realized we should have researched better.