An empire of sea-peoples?

[TheKillerBs]

It isn't so much that the people in question not claim land or establish strongholds, in fact they must. And with advancing technology they or their cultural heirs are to build canals and locks. What I'm focusing on though is the difference between a settled people and a nomadic people's ability to mass against an enemy, and yes that depends on alliances and communication between the different groups, such as fast scouting and messenger vessels.

You may also want to take a look at the nomadic conquerors of the Middle Ages, such as the Turks and Mongols. Yeah, they travelled by horse rather than by boat, but they also had the same problems of being nomadic warriors in charge of sedentary farmers

 

[Puddles]

Another point of inspiration could be drawn from the Kingdom of Portugal, 15th century. Their mountainous terrain gave them an advantage over invading forces by land, and also pushed them to become sea-farers themselves for expansion. Perhaps your sea-peoples have a capital city that acts as their power-base and cannot easily be conquered by land, (for example, it is encircled by massive mountains on all sides).