Translation:He does not go swimming because he is not well.
Placement of the tone mark
In syllables where the vowel part consists of more than one vowel (such as diphthongs and triphthongs), the placement of the tone is still a matter of debate.
Generally, there are two methodologies, an "old style" and a "new style". While the "old style" emphasizes aesthetics by placing the tone mark as close as possible to the center of the word (by placing the tone mark on the last vowel if an ending consonant part exists and on the next-to-last vowel if the ending consonant doesn't exist, as in hóa, hủy), the "new style" emphasizes linguistic principles and tries to apply the tone mark on the main vowel (as in hoá, huỷ).
In both styles, when one vowel already has a quality diacritic on it, the tone mark must be applied to it as well, regardless of where it appears in the syllable (thus thuế is acceptable while thúê is not).
In the case of the ươ diphthong, the mark is placed on the ơ. The u in qu is considered part of the consonant.
Currently, the new style is usually used in textbooks published by Nhà Xuất bản Giáo dục, while most people still prefer the old style in casual uses. And among Overseas Vietnamese communities, the old style is predominant for all purposes.