I guess that at least the jet ski speakers are designed to entertain the riders but yes, some other people nearby will hear the music. As has been said above, the jet skis go by and, at least where I am, they don't linger in one area. The sound systems on most pontoon boats are designed to entertain the passengers, not the neighbors. Wake boats on the other hand............
Last year I bought a one year old 28 foot boat. The previous owner had installed four rear facing speakers high up on the arch. The first thing I did was turn the speakers so they were facing down and the sound went into the boat, not across the lake. (Even though my taste in music is excellent)
I even shut off the stern speakers although if we were anchored in the right place I might tun them on.