Here's another vote for bathing well away from the lakeshore.

"Biodegradable" just means there's something out there which will eat it to some degree. It doesn't mean that the chemical compound in question will magically disappear, either quickly, or before it gets ingested by something which cannot metabolize it.

Intuition isn't much help in determining what constitutes "insignificant" either. Here's a link to a study done a couple of years ago on endocrine-disrupting compounds in Minnesota lakes:

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They found significant (typically 0.01-0.1 ppb) levels of DEET in ALL of the lakes and rivers sampled, including the supposedly-pristine (no surrounding development) lakes they had hoped to use as a reference.

It's a long report, and mostly about compounds other than DEET. But at least now you will know where pollyhumpers come from.

For the record, I am a chemical engineer, and no more of a tree-hugger than you would expect to find on these boards.