Well, the cells themselves can actually stand water perfectly fine
But the problem is the pigment that gives inklings their ink color, it reacts with water, fizzes up and causes pressure, causing the inkling to explode
Fun fact: if you threw a knife at one of my splatoon and osmosis Jones OCS, (like the one that skatez is) they'd probably just absorb it and it'd literally just be inside their body for a while unless they like spit it up or something