Isn’t it a bit silly for the Redwynes to have 200 Warships? How does House Redwyne finance and maintain what amounts to an enormous standing army, used less than a handful of times over the past 300 years? Is this realistic?

The Redwynes control an island that exports a highly valuable commodity and they exercise an unusual degree of control over their exports, so their standing navy earns its keep through escorting their trading fleets. 

And the Redwyne Fleet was used in the War of Five Kings, the Greyjoy Rebellion, Robert’s Rebellion, which adds up to 100% of the wars within ASOIAF proper, so it’s not like it’s not getting any use.