Not sure what advice you are looking for. It's 5.6 miles in either direction to the beach. You can grab a cab or uber and go north to Ft Lauderdale Beach and be dropped off at B Ocean or Ft Lauderdale Beach Park. Its 5.6 miles, or about 15 minutes.
You can grab an uber or cab and go south to Dania Beach and get dropped off at Dania Pier and its 5.6 miles or 15 minutes.
So, you have to decide how much time you are comfortable with at the airport and what you want to do at the beach. Either way you go, its a beach. Sand, water, etc.
Jet Blue probably wants you to check in within 2 hours of an international flight. No one can tell you whether you need to heed that advice or not. That is on you.
So, if you arrive at 8:50 and get to the curb to catch a cab by 9:15, you will be at the beach at 9:30. Then sit on the sand and stare at the water until 11 if you want and grab an uber back. Ubers might be more easy to find going north, but either way its doable.
FLL isn't a great airport to spend a lot of time in. Personally, I would grab a rideshare or cab and go have a nice breakfast at Grandpa's Bakery in Dania. Pig out on some serious home made pastries and hearty breakfast/lunch foods, and waddle back to the airport for my flight/nap.
Not sure what else you are looking for? How much is 90 minutes at the beach really worth to you? We can't say. It's not worth that much to me.