There was a post on the Ambergris Caye forum about sea grass along the beaches discouraging swimming. Is this also true of the beaches to the south---Placencia, and the southern cays? Is there sea grass off all the beaches? Is this because of the barrier reef? I guess it doesn't really matter to me that much. Just curious.

Can't answer your question - but love your board name

There are many many areas in Belize that you can swim directly off the beach or the boat and not have to step on sea grass (known as turtle grass, manatee grass, and shoal grass). I personally love seagrasses because of the many surprises it holds...lobsters molting, sea turtles and eagle rays feeding, and without it, manatees wouldn't have a place to graze.
Most folks paranoia over seagrasses are unfounded - and in most areas you can swim over it without touching it at all. Seagrasses are an integral part of the reef environment and without it, all those colorful fish that you see there would not have a place to to hide and feed while they are growing up to face the "big boys" on the reef.

No sea grass in the water when I stayed in Hopkins.

You obviously know the literal translation: "wind and water"

the village of placencia does not have much or any grass near unless the waves are kicking and then it's either not fit for swiming or floating near the top- futher north i.e. maya beach some will have small grass- the beauty of the grass is found in the above post. we tracked shellfish for hours. might have even dried one out and brought it home....enjoy dg