Greek ferries are the transport network, not a scenic extra — the islands have no other link. Seven crossings are bookable here, and the measured times set expectations better than a map does: Athens to Mykonos in two and a half hours, Mykonos to Santorini in the same, and Piraeus to Heraklion on Crete in nine. And a pair does not have one time: Athens to Santorini is five hours and the return is eight, because they are different vessels.
Piraeus is where almost everything leaves from. It is Athens's port, connected to the city by metro and suburban rail, and it is large enough that finding the right gate takes longer than you expect. Arrive earlier than seems reasonable, particularly in high season.
The other decision is vessel type, and it changes the crossing more than the operator does. Large conventional ferries have open deck space, lounges and restaurants; high-speed ferries save time and are markedly less comfortable in any swell, which the Aegean supplies regularly. Read the small print on the ticket too: most companies accept a digital ticket, but some still require a printed copy, and the gate is the worst possible place to discover which. Arrange the far end in advance too: on some islands, Santorini among them, it is better to pre-book the transfer from the port than to arrive and look for one.