Sentence:  Happily, however, he [the gnat] is born a swimmer and can take his pleasure in his native element, poising himself near its surface head downwards, tail upwards. Why chooses he this strange position? Just for the same reason that we rather prefer, when taking a <WSD>dabble</WSD> in the waves, to have our heads above water, for the convenience, namely, of receiving a due supply of air, which the little swimmer in question sucks in through a sort of tube in his tail. 
Meaning of the word marked with <WSD> tags:  An act of splashing in soft mud, water, etc.