Will order new prescription sunglasses next month. What features should I include in them for fishing? I'm aware polarized glasses are mandatory, but what else should be included?

Don't get the Transition lenses which go from light to dark and back. They are not polarized unless they get dark and they don't get dark when wearing a baseball cap or other brimmed hat.

Try on several frames and make sure they don't easily slide off your head.

I got Rx Costas in Copper base/green mirror. These excel in bright light like a sunny day on the bay. I got the 580g glass lenses which hold up much better than plastic lenses. I chose not to do any correction for far sightedness. I have to look over the glass to tie knots. To me that's not a big deal and it's not worth the additional expense progressive lenses cost. These glasses let me see into the water much better than my regular glasses and reduce eye fatigue.

Downsides are that the copper base filters out a lot of light. These glasses work better once the sun gets up a bit. There are other bases that can be chosen, but I'm not sure which ones are for Rx. Costa has a separate page online for what's available in prescription lense and style.

Costa has a lot of frame styles and sizes so you can find one that fits. They are expensive, but I great tool for the fisherman and great behind the wheel too.

I just did this myself. I got Costas in 580g green mirror. The copper base really helps to filter out a lot of "bad" light so that you get amazing color separation. I think in shallow water, you really want a copper base and green mirror. I keep raising and lowering my glasses to amaze myself at how much I can see with them. Glass over plastic was important to me for both clarity and scratch resistance.

I am an ABO certified optician and answer this question every day.

Fit of the frame (on your nose) and in general most important.
Costa is great and I love them, but they are not the only choice. Brown or Brown with mirror for inshore or freshwater. Gray or Gray with mirror for offshore or deep clear water.
"Transitions Drive Wear" is great for fishing and changing conditions. As PowerLizard stated regular Transitions are not that good, but Drive Wear is polarized brown (always) Indoors with no sunlight they are about 50% dark and in the sun get to 80% dark like all good sunglasses.
Neither Costa or Drive Wear are inexpensive options but both work very well in use.
Mirror or no mirror, if so which color? Silver is a neutral mirror and works well all around. Green mirror is better fresh and inshore, Blue mirror offshore.

Lens and mirror color matter based on what you are looking at. Shooting sporting clays lots of people use rose (pink) lens because it mutes the green of trees and the blue of the sky so that there is not a hard transition between open sky and across the trees. It also enhances the orange color of the clay pigeon so it is easier to see.

Fresh water is brown and green (dirt and algae suspended in the water) so using a lens that blocks those two colors that are being reflected back up at you, gives you better clarity. Same thing for the deep blue color of offshore water, Blue mirror helps cut down on that reflected blue light.

Thanks a lot for the input guys. Now I know how to talk to the optometrist after my cataract surgery later this month. Also plan to order from Zennioptical a pair of backup glasses. Oh, by the way I went to ACK Demo days in conroe this morning and bought a Native Slayer 12 LT Propel for freshwater fishing. Will take about 3 weeks for it to ship.