We have such a bench in which I wanted to maintain the natural wood finish after replacing the original wood which was rotten. Having observed how polyurethane varnish needs to be replaced every few years on a vertical oak surface (door) nearby, I decided it would be even shorter intervals in the case of near horizontal surfaces. I was advised to use a water based clear varnish, not least because it "breathes". Quite apart from the lousy finish, it was only two years until it had cracked and let in moisture to the point that the wood was discolouring. I truly dislike forever stripping and re-coating with anything, polyurethane or anything at all.
Elsewhere (outside the UK) I have come across a two part/component varnish which I have observed outlasts polyurethane by approximately twofold or more (so far). On the other hand, (in the UK) we have a small balcony which has a wooden floor. When I replaced it I used what I believe was an acrylic coating, water based. It went on very nicely and dried in a good, smooth finish - it has lasted more or less without fault perhaps twenty years by now. I applied either two or three coats. The big drawback is that it has a pigment in it, that ghastly red tint which in the UK passes for hardwood shade ("mahogany" or something, or just maybe "red pine") - but it is/was clearly a brilliant material. Don't ask me what the label said (manufacturer's name, type, shade, etc.), in any case it is probably no longer available.
If you want durability and you can find something along those lines I would advise you to try it, forget varnishes of any type - I don't think it was called a varnish of any type, more like "protective coat" of some sort. The pity is that no clear version was available, at least as I remember it. Alternatively you can use a neutral wood oil but that will need frequent re-coating, probably at least annually.