My boat is built out of birch ply, this is quite common in Scandinavia, I have seen boats being both 10 and 20 years (and maybe even 30) old built out of birch ply. Some of them in great shape and some of them in horrible condition. An example from Finland here (http://koti.kapsi.fi/~hvartial/white/white.htm).
Most people nowadays would never build out of birch since it is rot prone, but in my opinion if you find good quality ply and do a proper surface treatment there should be very little problems. My boat is glassed on the bottom and garboard, the rest of the outside is coated with three layers of laminating epoxy (more flexible than glueing epoxy) and painted with two part polyurethane paint, the inside is also painted with the same paint.
Last week I found a few pieces of ply in my yard, they have been there for about a year and a half, through sunshine, rain, sleet, snow and ice..... They have turned grey but I see no problems with the wood, tried to pull the ply's apart with my hands but was not able to....
Good enough for me!