Surprised you’ve found mixed reviews, I’ve only ever heard very positive things about Shell Bay.
I live in Bournemouth and ate at Shell Bay a few months ago, it was excellent.
You’ll struggle to find a more picturesque location anywhere in the country, fantastic views of Brownsea Island, Sandbanks and Poole Harbour. I was there on a date with my (now) girlfriend (Very romantic, especially if you cross on the chain ferry as a foot passenger)
It’s primarily a seafood restaurant, but there’s always a meat/vegetarian option too. Other good seafood restaurants in the area include Fishy Fishy (Dermot O’Leary’s place by Poole Quay), Westbeach (near Bournemouth Pier) and a branch of Loch Fyne (Canford Cliffs).