This episode was just pretty good. I really enjoyed The Evil Beneath, partly because of the Bahama Quadrangle gag. It was a good episode because the plot centered around all three of the main characters. Throw in one crazy (no, I shall say MAD) scientist that we've never seen before and algae men, and you have a great episode.
The problem was the other episode. Start off with the title. It is pretty misleading, because you think that Jimmy makes Carl a genius, but he just pretends to be one. Also, it revolved around just one character, Carl. I think, for it to be a good episode, all three main characters should have an important role.
In the end, only The Evil Beneath was great. But, because Carl Wheezer, Boy Genius was mediocre, the episodes combined only deserve an 7.7.