I confirm what DivingMeCrazy advise. I just made the test, where the underwater mode is assigned to either Fn key in one of MySet preset. The flash fires every time you trigger. When switching to another MySet where the underwater mode is not assigned, the flash doesn't fire anymore. The procedure to configure the E-M1 is provided in the Aquatica AE-M1 user guide (page 6).
Another (simpler ?) way to do so is to access the "Direct Control" menu by pressing "OK". Scroll vertically to reach the flash settings and you can choose between always on, red eye reduction, always off, 2nd curtain and manual value (info key + rear dial to change the value by 1/3 step from full to 1/64).
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