first of all, screw the peanut gallery and their opinions...you want a beach wedding, you have a beach wedding...if you are genuinely concerned, ask your wc or someone at the hotel/ceremony site if it will be crowded.
i can't speak for maui, but we had a pretty private ceremony. it was in front of a restaurant, but the restaurant wasn't busy b/c it was too early. there were actually people on the beach, still sitting in the palapas and our photographer took a picture of the couple as they read their books next to our ceremony...according to my guests, the couple didn't even look up! we had a microphone and speakers so everyone could hear.
we did have a few crashers at the reception...but my bm was on crash patrol and kicked all crashers out - even the ones i invited down from the balcony on the hotel!