The ATM at Belize Bank works with Canadian bank cards but that's not the issue. On weekends the machine tends to run out of cash and more than occasionally the power goes out and you can't use them at all.
Not all businesses will accept credit cards and those that do may add a service fee of 2% to 4%. Be sure to ask beforehand.
Your best bet, IMO, is to bring about $300USD in $20's and the rest of your 'cash' budget in USD travellers cheques.