Answer:
   
   There are two documented concerns for traditional Jews. First, the
   Shulchan Aruch (Orach Chaiim 326:7) discusses washing (presumably for
   hygienic reasons) in a river, and permits it provided that the person
   dry him/herself completely before walking four amot in a karmelit
   (i.e., the banks of the river). The Mishna Brurah (loc cit., note 21)
   points out that other (later?) authorities have ruled against washing
   in a river on Shabbat, because of the possible wringing out of the
   towel (an av m'lakha -- major category of activity forbidden on
   Shabbat). Hence, there appears to be concern about wringing out a
   towel after swimming.
   
   More directly, the Shulchan Aruch (Orach Chaiim 339:2) rules against
   swimming (recreational?) in a pool on Shabbat, on the grounds that a
   person might make a "havit shel sha'yatin" (raft, float), and that
   swimming might force water out of the pool and outside the eruv. But
   if there is a rim around the pool (so that there is no possibility of
   water spilling over the edge), there is also no concern that the
   swimmer will make a raft or float.
   
   There are also some other potential violations, but these are the two
   clearly documented ones.
   
   To Conservative Jews, this concern is not relevent today; when
   swimmers wish to use such devices in modern society, they purchase
   them ahead of time, and do not actually build one on the beach.