Went to new lake and now boat is yellow, wtf?

I normally trailer my boat (2k9 Supra 21V) back and forth to the river, which looks like chocolate milk, and wash it everytime I get back home. Last week we went on vacation to a lake up in MI and the boat stayed in the water the whole week. The water was super clear, you could see 8-10 down. But when i got home I notice that all the white areas of the boat looked yellow including the vinyl on the sun pad area. So I washed it with soap and water. . .nothing. I had to rewax the whole boat and wash the vinyl and use Malco leather vinyl cleaner. About 90% of everything came back to the original white except some areas on the vinyl. Any ideas on how to get this back to normal???