I was in the same boat as you not too long ago. I'll be 19 at the end of the month, and was having a lot of trouble with my Oakleys. I didn't want to spend much, but I know the saying "you get what you pay for." My Oaks fogged up really easily, and the polarization was terrible compared to my dad's Serengetti's.
So I started looking at Costas, and knew I'd be spending more than $150, which is tough for a college student. I knew the 400s would run me about $150-$175, and 580s would run me $200+. And lens color KILLED me. This is basically what I figured out:
-Blue: Open water, on bright days
-Green: Wade fishing, works on bright and grim days
-Copper/amber/yellowr: high vis fishing
-Grey: very sunny days
So I ended up buying Costa Del Mar Blackfins black frame with the green mirror 580 lens, which would have ran me $230 in stores. Got on eBay, found them for $175 shipped. It's hard to find the style you want, in the frame color you want, with your lens choice, and the correct lens color.
There they are