How to Protect a Backpack from Splashing without Plastic?

Question

The boat trip to Cabo in Tayrona National Park is apparently rough and the guidebook says to make sure to protect your baggage during the boat trip as it will get thoroughly splashed. Normally, I would simply place it in a large plastic bag except that Tayrona National Park has rules which include:

Entry of plastic bags and polystyrene objects is restricted.

What non-plastic cover can I use to protect an 80L backpack from splashing? It need not be protected from submersion, just regular splashing from saltwater for at least an hour journey.

Answer 1

Use a rain cover. They are basically nylon or alike fabric sheets, with a bit of sewing on the corners and have elastic sewn on the outer edge, which will tighten it around your pack.
Covers are available in a wide range of sizes and it does not matter if the fit is not really close, as long as the bag is covered and the elastic can reach the back.

It does not waterproof a bag, but it is certainly enough to improve the bags own properties to keep splash water out.

(If you can not find a photo, make one..., when you google -rain cover backpack- you will see plenty of them, but not within the rules for the site.)

Photos of the rain cover on my bike bag, backpack and cover out of easy reach, but the covers work the same.
Raincover with and over the bike bag. Photos by Willeke, can be used under creative commons rules.

Answer 2

Place your backpack in a large lightweight waterproof duffel-like bag, of the kind you'd use to avoid problems with straps getting caught in conveyor belts when checking in large rucksacks. I remember seeing Dutch backpackers use this method, and they had a lightweight cover/bag designed specifically to contain and protect their packs.