I think Jiraiya would win but it wouldn't be as easy as you say. Kisame has that huge water dome. Jiraiya has nothing on that scale. And yes Jiraiya has tons of chakra but there is a reason Kisame was called the tail-less tailed beast, he likely has more chakra than any one person. (Jinchurikki not withstanding)
Admittedly I may have oversimplified it. However we can both agree that in the end Jiraiya would be the victor. A few bumps and bruises but otherwise in good status
Also, about the scale of Jiraiya's techniques I kindly disagree. We can all agree that Manda was a very big snake. Not as large as Kisame's dome, but regardless. Even while Tsunade's drug was interfering with Jiraiya's ability to mold chakra he still managed to generate a swamp big enough to sink him:
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We don't know how much the drug was effecting him, but from what he says, clearly the technique could have been larger.
Now, considering Jiraiya at 100% health, using Sage Mode (which alone grants him a tremendous surplus in chakra), and the fact that Earth techniques are stronger than Water techniques, it's safe to say that Jiraiya could pump his Earth techniques with enough chakra to the point of even matching Kisame's dome. Granted it's a debatable point, but a safe one I think. It's a far less stupid argument that some other one's you see on these forums that's for sure
We know that elements that are weaker to another element can backfire on the user. It was explained by Yamato and we saw this with Sasuke vs Danzo:
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In theory, the same principle should hold true for Water against Earth