Having been in the law enforcement profession for nearly 17 years, I'd sure point out that the lifeguard didnt or couldnt verify the MPH. Humans dont come with calibrated eyeballs. Anyhow, if you didnt verbally admit any spped, it will be up to the guard to prove your MPH was over the limit.
85 % of moving violations are dismissed for defective instruments(the written ticket) or not being able to prove the alleged violation. Go to court, represent yourself "Pro se", put the guard on the stand, ask him logical questions, then specifically ask how he measured your speed....if he cant prove your speed other than visual observation, I'd say you could beat it.
Just my 2 cents worth, after all my years as a Deputy in kansas, but I am not familar with the laws within your state. During the summer, we co-op with Kansas Wildlife and parks for boating operations, I've never seen a speed ticket except in no wake zones ever be fined. Unless you can specifically prove "X" mph, its a matter of evidence submission...if he cant submit it, he cant prove it(the lifeguard).