Right, first thing I'd ask is did you notice any sound at all? That's usually the thing that separates the genuinely strange ones from drones or small aircraft. In my years driving overnight routes across the country I pulled over more than once to watch lights over water and the silent ones are a different category entirely in my book.
Also the reflection on the lake surface - did it cast one? Genuine light sources usually do, and if it didn't that narrows things down considerably.
Get your account written down properly before the details start fading. Date, time, duration, approximate size relative to something you could measure against like a treeline or the moon. Photos if you have any, even bad ones are worth posting up here. The blurry ones sometimes show things the clear ones miss, I've seen that happen with paranormal photography more times than I can count.
What part of the country are you in? There are known hotspot areas and it does matter for context.