Wade, a word whose root is found in both old Germanic languages and in Latin, used to be a strong verb in Middle English, “wade, wode, wad”.  “Swim” is among The Ten Thousand, but I note here that there’s no robust distribution difference between “wade” and “swim” in the text – there’s only one occurrence of “swim” after Chapter 9, just as there is none of “wade” after that point.

• 05.011 Still he did not dare to wade out into the darkness.
• 07.117 and shallow enough for me to wade
• 08.012 and we daren’t try to wade or swim.’
• 09.062 and waded ashore,

“wade, v.” OED Online. Oxford University Press, March 2015. Web. 9 May 2015.