I've used snorkel gear at my pond. IMHO the main improvement is from the use of a mask because then you can surface dive and get better visibility. That being said I don't think it will help that much to assess fish populations. Fish move very fast and randomly.

We swim in our pond ever year and our pond has been there for almost 70 years and we've never had any parasite issues.

JHAP
~~~~~~~~~~
"My mind is a raging torrent, flooded with rivulets of thought cascading into a waterfall of creative alternatives."
...Hedley Lamarr (that's Hedley not Hedy)