Using varnish (marine or spar or any kind) on horizontal decking that is outside and is splashed with water daily and rained on etc. does not work and when it starts failing, and it will shortly in most climates, is a real mess.
What one uses to preserve exterior wood like this is dependent on the type of wood. Most exterior benches are built from weather resistant woods like redwood, cedar, or pressure treated wood or tropical hardwood and do not require any applied finish--but some finishes can make them look more homogeneous and can add to their longevity. As a life time builder I have observed the varying trends over the years and most deck stains and sealers don't really add to the life of the decking much at all --choice of materiel seems to be the key element.
Basically you have penetrating deck sealers that soak in and and last a year or two, maybe a little longer; Penetrating preservatives like Copper Naphthenate or Jasco copper brown wood preservative that treat the wood to resist rotting rather than try to seal it from moisture; penetrating stains/sealers like cabots; and lastly film formers like pigmented stains, paints etc.
I have given up completely on any film forming product for a deck or something like a bonzo bench. Penetrating stains or deck sealers need such frequent reapplication I find people never actually do it--this leaves me to recommend, if you chose to slop on something to make it last longer I would go with a real penetrating preservative like Jasco copper brown wood or the like. Oh and tropical hardwood really does not ever need finishing and lasts and lasts but it is pretty darned spendy.