Well, it's not that tricky as Key West only really has two main beaches: Fort Zach beach and Smathers Beach. They are good but not stunning beaches. For that you'll need to go up the Florida Keys to Bahia Honda state park.
Here are your two Key West options:
BEST BEACH FOR RELAXING: BEACH AT FORT ZACHARY TAYLOR
- Bordered by Australian pine trees with tables - easy to get shade, makes a mice walk and also very pretty
- Not brilliant swimming but best beach swimming on the island
- Good cafe and toilets
- Can visit Fort Zach itself - big old fort and impressive
- Lots of variety along the length of the beach and so you won't get bored here
- Great for sunset
- Pay for loungers and other beach equipment
Here's my full review of Fort Zach beach here.
NOT SO GOOD:
- Not very big beach
- In certain parts of the beach it's quite shingle like underfoot (ie not perfect sand) so painful to walk and use
- Have to pay to enter as it's a state park - even if you just want to use the beach and even if you're only walking (but the charge is not a lot of money)
- No water activities available (like sailing etc)
BEST BEACH FOR ACTIVITIES AND SOFT SAND: SMATHERS BEACH
It's busy. It's louder. There are water sport adventure companies there. The sand is better. It's a pretty beach. It's free, unlike the state park access charge you need to pay to get into Fort Zach (even if you are walking, you need to pay).
But the swimming is not good because it's really shallow and lots of mucky looking seaweed. And there is very little shade.