The obvious way onto Kefalonia by sea will take you via Patras, into where ships sail from Venice, Ancona, Pescara, Bari and Brindisi, and it is understandably via this route that many of the hordes of Italian tourists arrive during July and August. Indeed, at the height of the season there are also sailings to Sami direct from Italy, avoiding the stop in Patras. The usual Patras - Sami crossing takes about 3 hours

As an alternative to Sami, you could drive (where applicable) the hour along the coast to Killini, from where a shorter crossing (1.5 hours) is available to Poros.

There are also regular sailings from Fiskardo to Lefkada, and from Pessada to Zakynthos. The ships used are small, though, so they don't sail when the wind gets up!

From Sami:
Sami - Patras
Sami - Ithaca - Astakos

From Poros:
Poros - Killini

From Argostoli:
Argostoli - Lixouri - Patras

From Lixouri:
Lixouri - Patras

From Fiskardo:
Fiskardo - Lefkada - Ithaca

From Pessada:
Pessada - Zakynthos

Summer Only:

From Sami:
Sami - Brindisi
Sami - Corfu