Charles 1 granted Appledore the status of freeport after he stayed there one night. Amongst other rights, this means that anyone can put down a semi-permanent mooring on the seabed.
I wouldn't think of a screwtype ; the moorings are laid by a chap in the garage at Instow, who has much experience of the the best type and size. Usually a socking great tractor wheel with tons of concrete.
There are a couple of cats which dry out in the vicinity of NDYC. There is a slight element of risk from 'visitors' if you dry out on the sand, but if you moor out on the really muddy bits, it's safer.
Also some risk of branches - even whole trees - coming down river if you have a big storm with lots of rain. There are lots of benevolent eyes around so make friends, even join that lovely club NDYC, and leave your phone number with them.
I guess you are going to be the biggest and smartest cat around, so a security device might be advisable.
(ex-NDYC member for several years.)
I think, therefore I am. I am, therefore I sail.