Hmm. I'll weigh in. We use spray on tanning lotions. Never have seen brown spots. I clean my seats after ever use with star brite vinyl cleaner and wax. My seats look as new as they did on day one and I'm 3 seasons in. Yeah, it's a bit of work, but I'm anal that way and I don't put towels down on the seats as it gets to be cumbersome especially with little kids.
The star brite cleaner and wax is great in my opinion.
You've got to be anal if you want your seats (or anything else on your boat) to remain new looking. My boat is 9 yrs. old (we've had it 6 seasons) and my seats/upholstery still look great!
I've been that way since my first truck when I was 17. I am meticulously a neat freak when it comes to vehicles and my boats. It's who I am and I'm Ok with extra labor in doing so My dealership salesman has bought for himself the last 2 trucks I've traded in for new ones. I don't baby them or my boat, but a little extra time in keeping them well kept goes a long way. Hehehe