Buying scuba dive boots is probably the easiest piece of scuba gear to buy. No sense making it more complicated than it is. This will be short and sweet.
There are basically 2 types of wetsuit boots.
Ankle High Boots- Ankle high scuba boots are usually made of thinner material and are great for warm weather diving where you're probably wearing a shorty style wetsuit.
They're easy to slip on and off.
Make sure they are high enough to protect your heel chaffing from your fins.
You can get thicker ankle high scuba boots for colder water and they're not necessarily a bad choice.
Be aware that you might have some exposed skin on your upper ankle when you swim, and that the increased water flow through the boots will let your feet get colder faster.
High Top Boots- High top wetsuit boots are a great all around choice.
If you're only going to buy one set of scuba boots, a thicker, 5-7mm, neoprene high top boot is good for just about anything.
You can get them with or without zippers on the side to make them easier to get on and off.
Fit- Wetsuit boots don't come in half sizes so always go a size up if you take a half size shoe. A size down may feel ok in the shop, but when the pressure at depth starts to crush the boot around your toes you'll wish you had a bigger boot.
Try your fins on with the boots. If it doesn't fit into the pocket of the fins, pick another boot. Or buy new fins.
Yes that means bring your fins to the shop with you when you go to buy scuba boots.
I'm sorry ladies but dive boots are usually quoted in men's sizes. I don't know why sizes aren't just quoted in inches or centimeters but they aren't.
Usually you subtract a 1 or 2 from your shoe size. E.g. A women's size 8 would be a men's 7(possibly 6)
Thickness- The thicker the material the warmer your feet will be. Your feet will never be too warm, but cold toes will make you want to end your dive.
Soles- Most dive boots have soles covered in rubber treads. If you do a lot of shore diving, a thick tread will be more comfortable and durable when you are humping your gear through the woods, over the rocks and across the beaches.
That's really all there is too it. Like I said, short and sweet.