It's really up to you. Personally, I would not care how much rain the lake gets. I would care about how much rain the yard gets. However, you may feel differently and that's certainly okay. As for UV and solar radiation, the readings will definitely be higher out on the dock as the yard would be subject to more shading.
Basically, you have two environments at your property. You're going to have to decide what you want to measure in each location. The good news is that with a WeatherLink Live receiver, you can have up to eight stations. So you don't really have to pick and choose. You can place whatever sensors you want wherever you want.
Example 1:
Yard: ISS with rain, temp, humidity
Dock: ISS with wind, UV, solar radiation
Dock: Temperature station (for water temp)
Example 2:
Yard: Full station with 24 hour fan
Dock: Full station with no fan or day fan (rely more on wind for air flow)
Dock: Temperature station (for water temp)