I use it all the time here in Southern California. Works really good at restoring the repelling characteristics of canvas. Sunbrella even endorses it with their canvas.
There vinyl protectant is the best I have ever used, I was able to even restore some old cushions to like new after a few treatments. I use it on my inflatable to protect it from the sun, 10 years and no sun spots at all.
I've used it on the Vinyl soft top on my Honda S2000 and it is still in good condition 5 years later. Other S2000 owners swear by it.
Before you ask about it in a marine situation it's as close as you can get without being on the sea. The car is parked outside on a headland of the north Cornish coast with the sea on three sides. Everything is covered in a thin film of salt crystals all the time, anything made of unprotected mild steel rots through in less than 2 years. It's as close to a marine environment ashore as you can get.
303 High Tech Fabric Guard is a tremendous product. Glen Raven mills has tested just about every "fabric treatment" on the planet and the only treatment they recommend for Sunbrella is 303 High Tech Fabric Guard. I had a loooong chat with the Glen Rave rep one day about this. Glen Raven makes NO money off this recommendation as 303 & Glen Raven are two separate companies. They make the recommendation to uphold the reputation of Sunbrella because 303 HTFG is simply the best product available to use..
If you have Sunbrella or another acrylic outdoor fabric use 303 HTFG. Do not confuse this product with 303 Aerospace Protectant....
303 HTFG is for fabrics not vinyl. IF you have vinyl, rubber, plastics etc. use 303 Aerospace Protectant but for Sunbrella 303 HTFG...
There were a couple of places selling it cheaper but with more expensive carriage. This company seems to be a main importer (apart from some of the hot tub companies who get it bundled with their products). When I called them and asked for a discount to match another supplier they gave me one, subject to buying a reasonable quantity.
FYI I have quite a big spray hood and it took about 700ml to do it.
I bought some from www.motorgeek.co.uk last year - Two off 32oz Fabric guard for £39.90 (total) plus £6.50 shipping. The web site today shows it at £20.38 for the 32oz bottle. Look for the manufacturer selection box on the left side about halfway down and select 303 products.
Worth checking out maybe?