We used Searious last Wednesday and I really enjoyed my guides (Ricky and Devon on the first boat but I can't remember the guys name at Lamanai itself). They didn't rush us at the site but hauled butt to get us back on time. If you have at least six people going, they will do a custom boat for you (at least they did for us!). It won't be a charter and they may put some others on it but you are more likely to have a smaller group.

Another thing to consider. Ask what days the cruise ships come into Belize and avoid those days. The last thing you want is to get stuck behind a ton of people.