The problem appears limited to the Galaxy S7 Active, a rugged model available only through AT&T in the US. The standard S7 and S7 Edge models have the same claims on water resistance and passed tests.
Consumer Reports, a non-profit organisation that is well respected for its product testing, has said that it cannot recommend the Active because it does not meet Samsung's own claims.
By contrast, Consumer Reports rates the S7 and S7 Edge phones as "Excellent" for their displays, battery life and cameras.
Maria Rerecich, Consumer Reports' director of electronics testing, said she was surprised the Active failed. She said Samsung's claims are usually "fairly good".
Samsung said it has received very few complaints and had subject the phone through strict testing for quality assurance, but "there may be an off-chance that a defective device is not as watertight as it should be."
The company said it was in touch with Consumer Reports to learn more about its tests.
Samsung says all S7 phones, including the Active, are designed to survive in up to 5 feet of water for 30 minutes. Consumer Reports says that after a half-hour in water, the Active's screen flashed green and other colours and did not respond to touch. Tiny bubbles appeared on the camera lenses. Consumer Reports tested a second Active phone to verify results and it also failed.
Rerecich said it is likely that the problems affect only some of the units manufactured, but "we bought two and they both failed". Both units were bought online.
Samsung says the Active can withstand drops of up to 5 feet on a flat surface, though Consumer Reports did not test those claims.
In the US, the Active phone costs $795, which is the same price as the Edge, and $100 more than the regular S7.