That is indeed the infamous "Quest" or Polybutylene piping with really horrible fittings. (Y'all can google the lawsuits, etc...)
It's abject garbage. Twalker has given you the solutions for "make it last a few more seasons" which are fine, if that's your goal. All uses of this stuff: houses, boats, RVs, have suffered many failures just like you've experienced. They'll continue as long as you have this stuff in your boat.
Me? I'm a pretty good armchair plumber, so I'd completely repipe with PEX-A. Take maybe a weekend or so including serious beer breaks after you source fittings(and the magic expansion tool). Will _never_ leak or go bad. PEX-A is amazingly reliable. I'd use the plastic Ts, Els, threaded adaptors on a boat over the brass ones. (weight and corrosion...) A C-30 doesn't really have that much plumbing so it's not a massive job.