I replaced most of our freshwater system this spring. Got some really great fittings at Home Depot. They are brass 'push-on' things and I used a 3/8" ID flexible plastic hose that HD had as well. I was very sceptical about the goods and was actually on my way to the Marine store to buy some hose and fittings there, but the fittings looked good and I thought "what the heck - if they don't work I haven't wasted thousands of dollars".
They worked so well that I went back for more and I'll be replacing the remainder of the old stuff as soon as the season's over.