plywood is stronger than any grade of 2X4's hands down. If you have a choice fir ply is stronger the spruce ply and only a little more money. For laminating the plywood together I would recomment in using a 2 part marine glue. As far as plywood delaminating, if properly sealed/painted there shouldn't be any problem.
I have used the marine glue in constructing various projects that I used on/in the water.