Since the cooling fans on the framework 16 is in a different compartment and not attached to the mainboard, I wonder if it is possible to waterproof the laptop by sealing the holes around the mainboard/battery compartment with some sort of putty and using gaskets around some ports. The putty would have to be heat resistant to seal the gap between the heat pipe and the enclosure, I think a silicone putty would suffice although I have never used it. The usb-c ports would probably be susceptible to salt water corrosion, so i think it would probably be best to put removable seals on them in the form of a blank expansion module with a silicon or rubber sheet to form a waterproof seal on the port. There would need to be similar rubber seals around where the fan module mates with the enclosure, especially where the pcie interposer is. Hopefully coating the pcbs inside the fan module and the lid with something like silicon or acrylic will suffice for waterproofing those parts. Speaker grills would have to be sealed but who needs speakers when you have headphones or perhaps a speaker expansion module?
i believe there is another way to water prof that involves a clear paint like application right on the board.
The fans are in a seperate compartment - the heat sinks are not. You need airflow between these compartments.