The museum is bit hard to get to from the center of the city, but easy if you came to Athen's via sea at the Piraeus port. You need to get to the port, and then take the 904 or 300 City Bus to either Plateia Fredyttou, or Tzaneio Hospital. A taxi can also take you there. It is close to the ZEA Marina.
It is cheap to get in (4 Euros, 2 Euros specials). The Museum is not large. About an hour will fully cover it. But it has a lot of cool artifacts, and especially ship models. The book store is full of good stuff, most in greek, but some in english, all in very cheap prices.
The electircal railway stop at Pireaus
The outside of the museum. The sea in the front, and across the Ellinikon-Faliron-Voula areas