When it comes to finishing your project, it is not as simple as it used to be, a good quality exterior house paint used to be the recommended option, however technology has moved on significantly and you now have many options depending on your desired finish, funds, skill level, and equipment available.
Two-pack products will give you a superior and stronger finish than most others, however they can also be the most difficult to apply and repair (often require spraying). Two-pack paints are a very hard-curing paint which means they can crack if there is movement rather than stretch like a softer paint. If you are confident with your application of the two-pack products you shouldn’t have a problem given all surfaces to be coated should be stabilized (epoxy/fiberglass over plywood). We predominantly spray professional grade two pack products in our workshop that aren’t available off the shelf, as we strive for an automotive grade finish, which is unfortunately out of reach for many amateur builders.
It is also worth keeping in mind that some linear polyurethane products can't be polished/buffed which can make life difficult if you do not have much experience with two-pack application and are prone to creating minor imperfections that could otherwise be polished/buffed out.
The most commonly used these days for marine applications by amateur builders, and an excellent middle ground, is a marine grade single pack polyurethane product that can be brushed/rolled and tipped for a great finish if done correctly.
Our recommendation for most applications would be a marine grade single pack polyurethane product, if available, applied using the roll and tip method.
When it comes to both paint and varnish, International paints can be used to produce top quality results.
If you have trouble finding a marine grade single pack polyurethane product, the next step would simply be a suitable marine grade paint/varnish. You should have no problem sourcing this easily at a local hardware or paint store. Bunnings even has some varnishes that we have used in the past to good effect, e.g. Bondall Monocel marine grade varnish, or Feast Watson Spar Marine varnish. Once again, varnish can be applied easily and produce first-rate results using the roll and tip method.
If you are considering staining plywood you will need to ensure you use an epoxy friendly stain. Behlen Solar-Lux is a good option, however you may find others. It is worth noting that producing a good and consistent stain finish can be tricky.