I think the display starts early PM but check the
Clacton Air Show Website for exact timings.
Yes there are restrictions on anchoring . A trot of buoys is laid about .5 mile offshore and you are not allowed inside of that line.. If you do a Police launch zooms out and moves you on. Its only worth going by boat if reasnobly calm and you can lay comfortably at anchor.
The display is supposed to take place between you and the beach , but in effect a lot of them stray right over the top of the anchored boats so its very spectacular (& noisy)