I landed at Barra last month in a Reims Rocket. It posed no particular problems apart from needing to have the conviction that the parts of the runway covered by patches of shallow water (maybe 10 or 15 mm deep) are actually firm. There are three runways all pointing towards the tower (AFIS) and marked by single posts at each seaward end. The take-off run seemed to be similar to soft grass. I felt it was important to use the soft field techniques as the Rocket has a lot of weight on a small nose wheel.
No overnight parking may be the official line, but there is a slipway allowing the aircraft to be pushed/taxied off the beach and above the high water mark. Check when you PPR. There is no avgas.
The aircraft needed a good hose down afterwards and sand is still appearing in odd places.
I believe thatb the sand caused problems with retractable landing gear on the larger Loganairs (Shorts?), so Twin Otters are now used.
Barra was definitely the highlight of an excellent long weekend -- you'll love it, Whirly.
I also have a very distant memory of a pleasure flight in a five-seater biplane from Southport beach when I was about ten years old. It either cost half-a-crown or five shillings.
AA