One solution I've just stumbled across is this: Kraken Sports
Universal smart phone housing. Fits my budget, and I do like my Samsung S8 picture quality. Definitely not a proper UW camera but will get the simple snaps done.
Anyone had any experience with it? If I get it I'll post a review.
I would only use an old phone for this. I wouldn’t trust my current phone which was expensive in something like this.