want to take the morning ferry from pireaus to mykonos with my family of four & enjoy the scenery.
what class of service would you advise to book? what actually is business class seating?

Assuming you're planning to take the Hellenic Seaways Highspeed, business class seating is separated from the economy seating and may have 2 seats facing two other seats with a table in between. Economy seating is like economy aircraft seating. You really don't need to spend the extra money for business class unless you want to play games or do something that requires a table.

didn't want to take the high speed rather the slower ferry to enjoy the beautiful views...

In that case business class seating is in separate lounge areas inside the ship. Economy class seating is the deck around picnic and patio-type tables with resin chairs. If you take one of the Blue Star fleet for a few euro more you can get an indoor economy aircraft-style reserved seat but you can still sit outside and wander around the ship.

Whenever I can I take the M/S Superferry II from Rafina to Mykonos and I prefer first class. It always remembers me my travels from Ostend to Dover. Because the Superferry is the old Belgian M/S Prince Laurent, sailing between Oostende and Doover.
G.