Here's what I'd get: ULA Epic

It's not a traditional packbag, being a roll-top drybag, but the assembled pack will carry identically to a good pack and importantly, ULA has very concise fitting options and know what they're doing.

Seam-sealed traditional packs, of which there are a few, are fine for eternally soggy climates but probably not adequate against actual dunking of any duration.

Cheers,
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--Rick