I will bet that 99% of the varnishes on the market including marine varnishes are polyurethane based. Originally varnishes were natural pine sap resins, oil and a petroleum solvent. All of that has been replaced with polyurethane today.
I also don't think that there is any difference between marine varnishes and the stuff you put on floors or furniture that you buy at Home Depot for half or less than marine varnishes.
A good varnish job requires meticulous preparation, five plus coats with light sanding in between and yearly top sanding and recoating and repairing any film breaks. If you do this religiously you can get 5-10 years before you have to strip to bare wood.
Another type of coating is Cetol and its clones. It is entirely different from varnish and is partially opaque or brown. It partially dissolves the undercoats each time you apply so it has a better ability to repair breaks because it does not leave an edge like hard varnish will. But it is softer and scratches and gouges more easily. The brown color helps it withstand UV. You can usually just keep repairing Cetol for years and sand and top coat when needed.
David