As I said in another thread, I think we can help each other by sharing stories, good and bad, about players in the boating market. Here are a couple of vendors I've had very good experiences with:
ProMariner -- I've had a couple of pieces of their equipment. Over the years I've asked them some fairly detailed tech questions about installation and configuration. They have always provided prompt and very detailed (and very helpful) answers.
Taylor Made Products -- I've had a couple of things replaced under warranty. One was a fender that was 11 years old that I didn't purchase - it came with the boat. They've been pleasant to deal with and provided replacements with no hassles.