I looked up the product manual for the IC-M304. It says, "Equivalent to IPX7 (1m for 30 minutes, except for cables)." Further translation from outdoortechnology.com: Protected against heavy splashing and rain. Must not fail or show water seepage when exposed, but not when immersed. IPX-7 Waterproof Standard. Protected against a short duration of water immersion. Max immersion for less than 30 minutes and at a depth of less than 1 meter.
Bottom line: The label on the M304 is submersible. The rest of the unit - not so much.