First thing on the checklist should be waterproof if you're going to be reading on a boat.
I have Pocketbooks and Kobos - I think the Pocketbook native reading software handles pdf better than Kobo but thats for novels; for technical documents you may need more. I also think you you'll want a bigger screen for those so I'd suggest you have a look at Pocketbook Inkpad 3 Pro or Kobo Forma.
These are both waterproof, with 8 inch (or thereabouts) screens. I prefer the shape of the Inkpad but if you're in the UK the Forma is on sale for £149 so that's a big selling point right there!
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