Slide - End In The Water, Not Above It?

You'll need to address this with your building department since most have strict guidelines on slides.

You also need to consider that kids are going to want to jump from the top of the slide since it will tall and near the pool.

I was on a build years ago just like that. Was a renovation no inspections. They had a 2ft deep nook and the guy owned a bodyshop they either made from scratch or veneered a slide with fiberglass cant remember. Had a water feed hooked up the hard part was the coping was notched for the slide it ended right at the water height. Gotta watch bigger kids sliding amd hitting their heads the codes exist for a reason

Something to consider...one of the "funnest" aspects of a slide is that brief airtime you get between the end of the slide and the water. You'll lose that. Also, with a long extension going right into the water, you miss out on one of pool / slide's most important roles....that of 'kid catcher'.
I have very fond memories of standing at the bottom of the slide to catch the little ones as they come down, letting them get a tiny bit more submerged as their courage and confidence grows . (Pro tip: Extra giggles can be had by catching and swinging them around in the air before touchdown.)
Sooner or later you get the word they want you to just stay there, but not catch them. Finally, they very proudly tell you they got this, and your catching services are no longer needed.

My suggestion would be that if you go forward with your plan, do it in such a way that it can be reconfigured and used as a conventional slide later.