"...diving is a very sketchy water sport." - No you can't say this. Sketchy has various meanings - the best is to check in a couple of online dictionaries (e.g. https://dictionary.cambridge.org/dictionary/english/sketchy) The most common meaning in my experience (UK) is vague, unclear, approximate, partially defined etc.. "She made a sketchy plan for how to study for the IELTS exam, but it needed a lot more detail." In the US (mostly), I think it can also mean: insecure, risky, unsafe e.g when talking about an area of town which might be dangerous. In the UK we might use the word "dodgy" in this context.

No, because sketchy has the meaning of “suspicious”, so that’s the right way to go. But really, you can’t use the word “dangerous” to replace the word “sketchy”. Because the sentence you used “diving is a sketchy sport” does not have the right meaning, because diving is not suspicious

While Charlie's answer is correct about some uses of the word sketchy, Casey is definitely correct that it is used this way. It is slang though. Casey's examples are very good, but I think when used this way it is often a feeling rather than a description - in the UK at least. "Crossing that bridge felt really sketchy" - "It really sketched me out". "Your driving is sketchy as hell" - "Your driving makes me feel very nervous". "That looked sketchy" - "I can't believe something bad didn't happen while you were doing that" It's definitely slang when used this way though.

No, it's not the same thing as "dangerous." Sketchy is used to describe a dark alley in a crime-infested neighborhood, or a guy that you meet who gives you a really uncomfortable and sinister sensation. Sketchy tells you that something isn't quite right, there is potential for danger, that you might be kidnapped, robbed, etc.

"Sketchy" is a little different from "dangerous". These are the most common ways to use sketchy: Sketchy can be used to describe a bad neighborhood ("it's a sketchy neighborhood"), or a suspicious-looking person ("that guy looks sketchy, stay away from him"). Also, it's used when something is made in a rushed way, or with bad quality, and it could easily break or hurt someone ("that motorcycle seems sketchy, I don't feel comfortable riding it"). It can also be used to describe a situation where something seems like it could go wrong ("don't walk through the park at night, it's kind of sketchy"). It's more common to say: "diving is a very dangerous water sport". But you can also say "diving is a very sketchy water sport". People will know what you mean, but most people usually don't use "sketchy" like that that. I think "sketchy" is usually used to describe an observation about how something looks or feels to you. Also, "sketchy" sounds more like slang, so it's better for when you're talking really casually.