My semi-educated guess would be that ATC will clear the meatbomber to climb and descend out of the way of lanes of traffic in the area, likely over the bay depending on which rwy is in use at YMML which should be safe enough.
Proximity to water would unlikely pose a problem the tandem masters will all be very experienced and used to landing in certain areas; wind is also taken into account and if it's blowing offshore too strongly, they won't land on the beach. They'll have lifejackets with the pax just in case tho.
However I would also have concerns with aircraft potentially flying through the DZ enroute to Brighton or towards YMEN, especially considering students who are struggling with high workloads (I know I would have likely missed the call during training), and would be interested to know how they are going to negate the potential risk?? A tandem master, pax and rig will make a mess of any aircraft if they end up sharing airspace.