I would have recommended an Android tablet to ensure full linguistic compatibility but given your waterproofing requirements it sounds like the Kobo Sage or second-hand Forma would be the closest alternative. I don't know for sure if KOreader can support biblical languages in all their glory but since it allows for external fonts and dictionaries it will in all likelihood afford you the flexibility to do so.
I would look for the KOreader main thread and ask about the languages there.
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