Roy, your young man has gravely misled you! The solvent type polyurethane varnishes and traditional yacht varnish are still readily available in small tins although not always stocked by the main DiY stores. Ronseal, Blackfriar, Rustins and Wilko are the main brands. Local hardware stores are the most likely stockists.
The water based acrylic stuff you have has two main potential drawbacks:
It tends to be water resistant rather than waterproof and, as Howard has already said, if immersed in water has a tendency to bloom from water absorption.
It does not always bond well to the surface to which it is applied. Probably OK over a matching primer but not over something like an enamel or car paint topcoat where it can just constitute a film covering. As it is soft, it is only a matter of time before you get a knock or abrasion and then it will start to peel off. Been there, done that!
Yacht varnish is intended for use on timber finishes and not as a top coat to a painted surface. It is a flexible covering intended to allow for movement of the wood and has a pronounced yellowish tint which will alter any underlying colour.
Interior polyurethane is the best to use as it is more transparent than the external variety which includes UV filters which also give it a slight tint. I have never known it react with anything it is applied to as long as the surface is fully dry.
Colin