Crews spotted the unfolding emergency at around 7.30pm yesterday as they were taking part in launch and recovery training outside their lifeboat station at Central Pier.
A lifeboat was quickly launched to rescue the pair who are thought to have gotten out of the depth while playing in the water.
The girls, who were both thought to be around 11-years-old, were struggling to stay afloat when the life boat reached them, says the Coastguard.
A spokesman for the RNLI said: "Our RNLI volunteers were in the middle of their weekly training exercise when they spotted a real emergency unfolding right in front of the lifeboat station.
"Some of our volunteer shorehelpers heard two young girls calling for help. The girls had got out of their depth whilst playing in the water and were struggling to stay afloat.
"Our volunteers quickly launched one of the station's D class lifeboats and took the girls safely back to shore.
"Please never swim alone and always make sure someone is watching you from the shore and is able to call for help if you get into difficulty."
Ambulance services met the rescue party and took the girls to hospital.
The girls are not thought to be seriously injured.
Did you witness the incident? Get in touch and let us know.
Always call 999 and ask for the Coastguard if you spot anything that gives you any cause for concern.