Remember too that high grade plywood has more plies and are therefore more stable. It's not just about voids although that matters too. More expensive plywood will have better glues, better quality wood and more plies. Some cheaper plywood will have only 3 plies for a 1/2" sheet. Marine plywood will have more plies - probably 5 for the same sheet. This tends to be true for cabinet grade plywood as well. Cheaper plywood requires more structure to keep it from warping or changing shape. Think about longevity also and the conditions in which the plywood will have to perform. I don't generally fish in rainy/stormy weather and keep my boat covered so my plywood doesn't get soaked. I assume it will last longer than if I was in the rain a lot. Here is a description of marine plywood from Wikipedia...
Marine plywood is specially treated to resist rotting in a high-moisture environment. Its construction is such that it can be used in environments where it is exposed to moisture for long periods. Each wood veneer will have negligible core gap, limiting the chance of trapping water in the plywood and hence providing a solid and stable glue bond. It uses an exterior Water Boiled Proof (WBP) glue similar to most exterior plywoods. Marine plywood is frequently used in the construction of docks and boats. It is much more expensive than standard plywood: the cost for a typical 4-foot by 8-foot 1/2-inch thick board is roughly $75 to $100 US or around $2.5 per square foot, which is about three times as expensive as standard plywood. Marine plywood can be graded as being compliant with BS 1088, which is a British Standard for marine plywood. There are few international standards for grading marine plywood and most of the standards are voluntary. Some marine plywood has a Lloyd's of London stamp that certifies it to be BS 1088 compliant. Some plywood is also labeled based on the wood used to manufacture it. Examples of this are Okoume or Meranti