To be honest, under normal circunstances, a water user would loose to an earth user.That's simple math. However, when it comes to Kisame, that logic no longer works.
If samehada absorbs the chakra out of the explosive clay, the explosive clay no longers becomes explosive, no ?
Furthermore, I doubt normal explosive clay would work well underwater. If Kisame uses his water dome jutsu, the chances are Deidara loses really quickly. I'm not even saying Kisame goes full-shark mode on, just a regular Kisame underwater is quite the opponent. Deidara wouldn't be able to hold his breath under water for long periods of time and if his clay doesn't work underwater, like I said, he's literally screwed since he can't do anything about it and it's just a matter of seconds before he drowns.
In case his explosive clay does work underwater, he may be able to escape the water dome, but he will be terribly exhausted to the point where Kisame can give his final blow to finish the fight. I mean, the only option I see is distracting Kisame with small but fast explosive "fishes" that either make him stay in his position or move backwards( this will move the dome backwards, making it easier for Deidara ) while Deidara creates a bigger fish to get the hell out of the dome. In case this does work, I don't see Deidara getting the upper hand during the battle under these conditions so it will end up in a draw, if not a win for Kisame.
In case the explosive clay does not work underwater, Deidara simply can not win, whatever he does. Even his ultimate art, his self-sacrificing bomb, is simply a chakra bomb which wouldn't be useful against a chakra absorbing opponent, right ?
Of course, even after all this, Kisame still has plenty of tricks udner his sleeve that can utterly destroy Deidara really fast. If Kisame goes full-shark mode, he is simply unstoppable. He would simply need to touch Deidara once to leave him with barely any chakra, if any chakra at all.
So, after going through this facts, I think Kisame would win.