Shields of the Athenian Marines?

[PMBardunias]

In my imagination individual marines likely crossed from ship to ship on their own legs and that a degree of individual combat must take place

Unlikely, because the defending marines are under no such constraint and would surely meet any attempt to cross with a group. So going back to the original question, the only time an aspis might be less than optimal was the moment one was crossing over (if the contact between ships was so narrow that only one hoplite could cross). For the rest of the battle, an aspis would be perfect, as it would be for the marines upon landing.

As Stephanos points out, this type of battle on shipboard was a minor point in the naval warfare of the Greeks. Perhaps as interesting is when hoplites attempted to force anamphibious landing. Things did not go so well for Brasidas in this regard, for example.