Sorry for the slight delay.

The most common parasiste that dogs and people too can get from a water scenario - lakes, streams, puddles - is giardia. This is a protozoal organism that can live in the water and if ingested, live in the gastrointestinal tract of your pet. Once there, it can cause nausea and diarrhea.

It is an organism that can be diagnosed by examining the stool under a microscope or with a swab ELIZA test of the rectal mucosa - it looks for the giardia antigen. Both of these are fairly simple and not too expensive of tests - honestly they shouldn't be too expensive $20-40.

As far as you doing something at home for the diarrhea. You can feed a high fiber or bland diet. The one at home diet that I recommend a lot is boiled rice and chicken - 1/2 cup to 1/2 cup. 1 full cup per 20 pounds of pet.

At a veterinary office, they have very good high fiber foods that I use a great deal to help get the stools to firm up quicker. As far as the actual treatment for the diarrhea. If it is giardia, they can prescribe an oral medication - fenbendazole to treat it - 5 days is the usuall treatment course and not expensive either. If there is no giardia - then a lot are treated with a course of metronidazole - an antibiotic to help get the gut flora back into check. This is also not an expensive antibiotic.

Here is a great link on giardia.
http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=2386

Dr. Bruce