The term "ジョボジョボ" (or じょぼじょぼ) is onomatopoeic, capturing the sound or action of water flowing out steadily or pouring out in a continuous stream. It's not directly about being soaked but more about the action of filling something with water or a steady flow of water. Given the context of filling the roof with water for a science experiment and the character's reaction while observing his foot, it seems like the dialogue is emphasizing the impressive nature of the water filling or flowing over the place.

Given the above, your Attempt #2, "Filling the whole place with water is great!" is closer to the intended meaning. The character is likely expressing amazement at how effectively and extensively the water has filled the area or is flowing, rather than the sensation of getting wet.

For your reference, I've provided a link to a sound library that offers a WAV file featuring the sound of water flowing, described with "じょぼじょぼ" in its description.