Congrats on the upcoming wedding! You have chosen a picturesque location!
I can't comment on the Placencia/Hopkins area, but the rest of the above comments are all true - some sandy beaches, some with seagrass, some better to swim off pier. Whichever way, all are great!
Our family always wears the inexpensive water shoes while wading. If your wedding is in November, they should be on sale after the summer!
Our kids don't particularly mind the grass, but it is just easier to put the water shoes on first (before any complaints arise).