I had the pleasure of traveling to Belize last week and stayed at Maya Beach Hotel. My friend and I took the Monkey River tour. We loved it but since I struggled to find tips before we went, I thought I'd add my own here for everyone to come.
First, I was grateful I wore long pants. The day we went, it was dry back at Maya Beach, and pouring rain all the way through the mangroves and in the jungle. We didn't mind the rain ( we're in the tropics after all), but again, long pants saved us. Wear a rain slicker. Also, wear grippy comfy shoes. It was super muddy and slippery in the jungle. The vines have prickers so there's no holding on if you're falling down. Just be prepared. Aside from all this info, it's definitely worth it and such an adventure.
Also if your tour stops for lunch at the Black Coral restaurant, you'll be able to use a clean bathroom at the restaurant. You'll stop there when you first get to Monkey River village.to make your lunch order and when you return from the jungle to have your lunch. The lunch was good! Re: bathrooms, I don't know if they're available for folks not stopping for lunch- they might be--I just can't promise anything. But they are there which is nice.
Bring DEET into the jungle with you. Some of our party needed some more and it was nice to share. I applied DEET lotion in advance, but was happy I had some spray w me too. Lots of bugs, but w DEET we were well protected.
Those are my tips. Do the tour. It's an unusual experience. Hope this helps.