In general the best plywood (or any material) for boatbuilding is the one that has the best strength / weight ratio, especially for a boat is designed to travel at higher than planing speed. For birch plywood this ratio is extremely good, so because of this birch plywood is one of the best materials available. Birch plywood is the stuff that airplanes were made of before they started making them from aluminium. For a boat that is designed for less that hull speed the extra weight does not matter so much as the extra power requirement for additional weight is so small. But in general the faster you want to go the more weight matters.
The basic problem with plywood and wood in general is that many times the plywood manufacturer does not know, or does not tell the exact values to be used for calculations for sufficient strength. Basically you cannot calculate how thick plywood to use if you do not know the strength of the material you use. The strength of wood depends very much of the direction of fibers, so the composition of plywood veneers is important. Also because the strength of wood depends highly on the moisture content of the wood, the manufacturer would have to have good control of the whole process of manufacture and storage in a consistent manner in order to be able to be sure about the quality and specifications of his product. That is not possible without enough knowledge and quality control of the whole manufacturing process. For plywood suitable for boat building you also need to use glue that holds its strength in constant moisture and does not contain voids. There are several standards that contain definitions for these matters and also for the quality control.
Because there are so many things that affect the strength of plywood smaller manufacturer dare not publish their specs for the plywood they manufacture so they cannot be sued for making unsubstantiated claims. I am posting here the specs for Finnish birch plywood. I know that there are a few big
Finnish manufacturers that export to USA also. Here is more info http://www.woodproducts.fi/wisa-birch-plywood
. Without specs based on proper study the strength of wood is pretty much guesswork. Thats OK also if you use big enough safety margins. Without better knowledge the general rule for wood strength is that the heavier the wood the stronger it is. But this a big over simplification.
Also consider the following:
Birch Plywood has about the same strength/ weight ratio as Aluminium. That is about 5 times better that for Steel and about twice as good as for Glass Reinforced Polyester that most boats are made of.