I’ve always used “the cheap stuff” and have been very happy with it. Helmsman spar urethane is less than $20 a quart, easily found everywhere and that should do the job. Prep is key, of course, and a light sanding with 220 between coats.
A few random thoughts from a veteran marine varnishing dude.
Preparation is the key to a good job. I usually start with 60 grit sandpaper in my Makita reciprocating palm sander, then 100 grit and finally 220 grit. Fill dings with the wood filler of your choice at the 100 grit stage.
Bare wood soaks up varnish more than adding a coat of varnish over existing varnish. Each coat uses less and less varnish. I use 220 grit sandpaper and steel wool between coats to dull up the surface enough to get good adhesion. From bare wood to finished job usually requires 4 coats on bare mahogany blades and 3 coats on bare ash tillers/tiller extensions. My favorite varnish these days is Interlux Compass Clear but it is hard to find due to COVID-19 induced supply chain issues. So like many others, I am becoming a fan of Minwax Mariner Gloss Spar Varnish, less expensive and readily available at most decent hardware or paint stores. Buy paint strainers at a paint store and strain varnish from the can to a clean varnish bucket before use.
Be sure to get all the dust off surfaces before applying a coat of varnish. Blow the surface with compressed air then use a tack rag (a clean rag soaked in solvent and a little varnish left to partially dry) to get the rest of the dust. Hang the parts as shown in the attached photos.
Start with a quart of your varnish of choice and thin it a little with the recommended amount of thinner. I like single-use foam brushes, but use a 4" single-use varnish roller and brush (roll and tip method) for varnishing 5 or more items at a time; twice as fast, better coverage with less sags/curtains and missed areas.