My Lazy-Boy Recliner has the same black wet marks - but it is 4 years old and has served us well in those 4 years.
The same stain on a brand new couch would produce a different, and slightly louder, result.
However, please learn from my mistakes:
First - read all the information in your purchase statement and everything that came with the couch - you may have some type of warranty that you are not aware of.
Then- Look at the couch tag (it is white and may be on a cushion or else on the underside of the couch). It should have a code for what can be used to clean it. http://www.apartmenttherapy.com/chicago/how-to/how-to-read-upholstery-cleaning-codes-057653
is one website that explains what the code translates to.
If it is cleanable - do so as soon as possible. Use a professional if that is an option. The longer you wait, the more set-in the stain will become.
As for the kids and wet bathing suits - for that I have no suggestions - other than plastic covers for the couch...(which are so uncomfortable that no-one would ever sit on it!)