I have had problems with acrylic varnish although admittedly it was the standard stuff you buy in DiY stores. I found that it dried to a film on the hull which looked OK initially but if the film was broken it started to peel off suggesting that it had not properly bonded to the underlying paint surface. There was also a tendency with extended immersion or in a damp atmosphere for it to go milky although that did disappear when it dried out again. You may not get these problems with the specialist acrylics though. (But they are not intended for situations where the surface gets wet regularly!)
I use Ronseal Matt or Satin Interior Polyurethane which can be thinned down for airbrush use. This is what Dave Abbott uses for his models and if it's good enough for him, it's certainly good enough for me. It's tough and adheres well although it isn't so easy to get hold of as it was. Blackfriars do a similar product which is probably just as good, I've had no problems with that either.
You can of course use the exterior polyurethane version but this has UV filters which give it a slight yellowish tint. Not a problem with planked decks or dark hull colours but it is noticeable over white. The interior stuff should be perfectly OK unless you leave your boat in bright sunlight most of the time.
Colin