Found this place when looking for dive sites. Virginia Key Outdoor Center has kayak rentals for $20/hour instead of the $50+ you pay on Miami Beach. Way less crowded and you can actually see manatees in winter months.

It's at 3801 Rickenbacker Causeway on the way to Key Biscayne. Also rent paddleboards and have a small beach area. Staff knows the best routes for spotting marine life. Cash only which is annoying but rates make up for it.