The owners have asked me what they should do to clear the algae since this
is used for recreational swimming and SCUBA diving.

If I advise them to plant hardy water lily will this help? Water hyacinth
would be expensive for so large an area and in Zone 5b.

That would make swimming unsafe.

Throw about 8 bales of straw , still tied in a bale, into the
pond...put a couple around the edges so they are half submerged.