Marine Ply is one of several grades of WBP ply
WBP means it is made using a glue that does not fall apart when it gets wet, even with hot water (ply before this invention used to delaminate and curl up in thin sheets)
Marine Ply is a higher grade, it has hardwood veneers which are more durable and expensive than softwood, and is not supposed to contain certain defects such as voids and knots.
If you are not building a boat, you can get a cheaper grade of WBP ply. IMO the hardwood faced (somewhat like mahogany) looks more attractive than birch, which is whitish with little grain, and tends to develop surface cracks. It can be dyed, and stained or varnished if you wish.
Cheaper grades of ply have more surface defects such as knots and joins. If you are able to find a competent joiner to build this for you he will understand the grades available and will know local sources and the quality and value they offer.
The edges of ply are the weakest part, and can allow water to enter, damaging the board, or can splinter off. They need at least to be sealed with paint or varnish, or can be edged with durable hardwood strip.
Some vendors will tell you they are selling marine ply when it is in fact a lower (cheaper) grade, and less knowledgeable people may use the term loosely and incorrectly.
Here is the UK spec
The term, and the specification, are different in America, where I understand softwood veneers can be used.
There are modern international standards and grades which I am not familiar with.