Where would the officers for a fleet like the Redwyne’s come from? Second sons and lesser lordlings? Or would the navy be a bit more meritocratic as opposed to the strict and established structure of a land-based army?

Why just second sons and lesser lordlings? 

The Redwynes and their vassals live on an island, so the cachet of the navy would be much higher than on the mainland of the Reach, where cavalry is king. Thus, you’d expect Arbor lords to command squadrons or divisions (under their liege lord who commands the fleet) and lesser lords and knights to command individual ships.