Trademark infringement occurs when you use a trademark name in a way that confuses consumers about the source of some goods or services. If you use a word that happens to be a trademark in a context completely outside the context of the mark as an identifier for a business, there's no infringement. Just because "Apple" is a trademark for a brand of computers does not preclude you from talking about pieces of fruit called apples.
Trademarks are also domain-specific, so a landscaping service and a restaurant might have the same trademarked name with no issue: there's no possibility of confusing the two businesses, despite their identical name. However, trademarks found to be "famous" in some jurisdictions enjoy a cross-domain trademark monopoly: you cannot, for example, sell bicycles under the famous name "Coca-cola" despite your bike business having no relation to the soft drink industry.
Outside of "Jacuzzi" as a trademark, the word only has signifance as a person's name. (It is named after the company's founders.) Since your product has no association with any person named Jacuzzi, but does appear to be associated with hot tubs, the potential for confusion is abundant. Your use of the Jacuzzi name in association with a hot-tub-related app clearly associates itself with the Jacuzzi Brands company without authorization.
It is possible for a trademark to succumb to "genericide" whereby the legal status of a trademark is lost due to widespread use of the mark to signify a generic category of products, rather than a specific producer of a product. For example, "aspirin" and "escalator" used to be trademark names, but now are no longer protected as trademarks. It is exactly out of fear of genericide that companies like Jacuzzi aggressively litigate generic misuse of their brand names.