the beach in Maya Beach (8 miles north of downtown Placencia)
it is calm like this 50% of the time at best. the water is real shallow, 5ft. deep out about 5o yrds so it gets churned up easy.
but when it's like glass, it can't be beat!

some other M.B. area photos here
http://www.flickr.com/photos/8797121@N07/sets/72157600368695768/
i have not been to A.C. for 15 years but i remember concrete seawalls. none of those on the peninsula. you can walk into the water everywhere.
i also remember A.C. being more crowded 15 years ago than Placencia is now.
that means there will be much more going on, more options.
the reef is further away so snorkle/dive trips are full days trips with lunch on a caye somewhere. usually a bbq with another snorkle stop after lunch somewhere on the way back.
access to most other mainland trips is obviously easier. the one drawback is to get anywhere, you have to travel the same section of bad road each time to reach the southern hiway. (this if you rent a car and do yourself. some tours leave from Mango creek which is a boat ride from Placencia.)
the road is now a super hiway from the point all the way to M.B. and they are working on the rest.
both location have their good and bad,,,see them both!