I admit total Victory was a near thing but total Conquest seems to have been impossible, since it would have required that those territories had not have any revolt from the subjugated people, and while elitist appointments are good for the empire, the fact that a sea dispersed territory could be controlled by what was essentially a land empire seems to be very hard to believe, so was the sea one of the reasons Persians could not properly defeat them in war, as contrary to a single battle where the Persians seemed to have the upper hand?
I don't see how you would reach this conclusion, given that their control of Phoenicia and Ionia gave Persia the largest, most advanced fleet in the Mediterranean. Despite its frequent revolts, they were able to retain their hold over Cyprus for most of the Classical period. The fact that their core territory was landlocked surely doesn't affect their ability to conquer places on the sea; historical empires don't follow computer game rules in which "land empire" or "naval empire" are fixed attributes giving particular advantages or bonuses.