Any kind of varnish is going to need regular maintenance and it's not a question of if it will need maintenance but rather when. A few years back Fine Woodworking reviewed varnishes and they gave the nod to Epifanes (it's a European brand available at marine supply stores and expensive). I haven't used it but I would suspect that it has the traditional yellowish hue of a marine varnish.

I've used General Finishes Exterior 450 and was pretty happy with it. It's water based so it dries quickly (multiple coats in one day) and cleans up with soap and water. Can be brushed or sprayed. There's an additive to slow drying time and one to increase durability. Unfortunately it hasn't been on the project long enough to make an accurate assessment of durability but so far it looks pretty good (exterior wood door, northern exposure, USDA zone 5b). Dries clear.

Frankly, for something like a rocker I'd use a deck/fence stain. Generally speaking, the more opaque the finish the longer it will last. I don't know what your mosaic's frame looks like but if there aren't a lot of nooks and crannies, I'd give the General Finishes a shot.

The link below has an interesting suggestion to use deep tint exterior oil based paint without the pigments. I haven't had a chance to try it, but I hope to in the near future. If nothing else, reading about the testing methods will have you in stitches!

Here is a link that might be useful: Exterior clear finish