Im an engineer and “carpenter” (come from family of carpenters and have helped my dad for a number of years. I know a decent amount in the trade)

This needs to be tied to the ground. I’d set 4x4s at least 2-3’ feet into the ground with concrete for the post. The 4x4 (?) joist you have the girders (what the kayak will rest on) sitting on could be switched to 2x6s and elevated off the ground.

The beam on the front and back which support the roof need to be at minimum a double 2x6 to support that span and roof load. The rafters are fine to be 2x4s. You need a barge rafter to hold the rafter ends in position. The over handing decking in the end with no rafter is no bueno. Decking cannot go unsupported for more than a few inches

The braces are fine. I’d probably add a short one on the side to tie back to the middle

Feel free to pm me for any other questions! Or reply to this