The Redwyne fleet, I think, is a special case. The Arbor was taken fairly recently, so it stands to reason that their fleet would've been damaged (hence, it's at 50/200 ships). However it can be rebuilt back to full capacity at 200 ships, at a price of 100,000/50 ships (same as a +3 castle or so, I believe).
In any case, the Ironborn still have more ships than both their nearest competitors, the Reach & Westerlands combined do atm (730 total to 290 & 140) so it's also a bit of a balancing issue, more important this time around since we're all independent kingdoms.
EDIT: Oops, saw that I'd failed to add it earlier, my mistake (Internet clogged up & ate my edit lol). I had proposed a special 75-ship levy for the Iron Isles, bringing them up to......600 ships. Which combined with Bear Island, Fair Isle, Arbor & those Riverland provs, adds up to the 730.