Will opening it up make it not waterproof?

While one can never guarantee the water resistance of electronics after disassembly, you should be ok as long as you put it back together before you expose it to water!

By ‘open it up’, I assume you mean disassembling the device as opposed to opening the SD card / battery port. iFixit has a page for this device that links to several repair guides that will take you through step by step disassembly. Looks like people have done all sorts of things, including removing the motherboard! If you are talking about simply opening the SD / battery port, you should definitely not do that while there is any water on the device.

This device is rated at waterproof grade 8 (IPX8), and is capable of shooting underwater at 10m for 60 minutes (see the manual, page 27).

Have fun out there!