"Depriving prisoners of drinking water as a punishment for participating in a hunger strike could result in prison authorities being responsible for the rapid deaths of the hunger strikers due to dehydration," the human rights group said in a statement issued in Bangkok.
The 15 political prisoners started their hunger strike on October 26 to protest the lack of reductions in their sentences, after reductions were granted to common criminals.
Eight of the hunger strikers have been placed in Insein's notorious "dog cells", windowless rooms about 3m by 2m, Amnesty International said.
On November 1, two of the hunger strikers were reportedly hospitalised.
Myanmar's treatment of its political prisoners is closely monitored by Western democracies, who have set the release of an estimated 2 000 inmates as a condition for normalising relations with the regime, which is now under a new government.
On October 12, the government freed some 7 500 prisoners, including more than 200 known political prisoners.
The release of all political prisoners is also one of the conditions laid down by Aung San Suu Kyi's National League for Democracy (NLD) for co-operation with the new government, which came to power after the November 7 general election.
Labour Minister Aung Gyi hinted last week that the government was likely to free more political prisoners soon.
"We will not stop, but nor will we jump with both legs," Labour Minister Aung Gyi said after holding talks with opposition leader Suu Kyi in Yangon.
Suu Kyi has headed the NLD since it was founded in 1989. She has spent 15 of the past 21 years under house detention.
Her latest jail term ended on November 13, six days after the general election that brought the pro-military government to power.