Actually chain tensioners are much happier in salt and muck because they don't have much in the way of moving parts, unlike a derailer which not only has the chain passing through an S-bend but--much more importantly--needs to be able to smoothly move laterally.

The real issue is that it looks like the Cypress uses a 135mm rear triangle spacing. Grab a ruler and check it out sometime to be sure. You'll want to measure between the inside faces of the dropouts. This has been the standard mtb spacing for many years and is the standard for 8/9/10-speed freehubs as well.

Unfortunately, the Nexus-8 only comes in 130mm, I believe. It's not clear to me if you could add spacers to space it out to 135. I've never seen one in person.