Should I be worried
There is nothing in the picture for reference to size; therefore it is harder to determine which spider it may be. The picture is also a little dark so it is difficult to determine the exact coloration on the spider. This spider is not a spider that bites people. It appears it could possibly be a fishing spider (depending on size). This type of spider is semi-aquatic and likes to be near water areas. It also could be a commonly found wolf spider. This group of spiders are ground dwelling spiders that can find their way into pools as they move around. Neither are cause for concern and I would not worry or make any pesticide application. You may see another in the pool sometime later in the summer. If so, remove to nearby area and things should be fine. Spiders are fantastic generalist predators that help keep pest populations suppressed in gardens and landscapes.