After hearing cries for help coming from the water, members of the public in Bembridge called the Coastguard.
This may have saved lives, the RNLI crew has said.
The RNLB Norman Harvey (D-778 Inshore Lifeboat) was launched at 11.28pm last night (Friday) following reports that cries for help had been heard from a motor cruiser moored at the visitor pontoon in Bembridge Harbour.
Bembridge Coastguard Rescue Team were first on the scene and found two casualties, one of whom had apparently fallen overboard and hit their head and another had gone into the water to rescue them.
By this time both had returned to the motor cruiser.
The Norman Harvey arrived on scene at 11.45pm to assist the coastguard, and an ambulance arrived at 12.10am.
A spokesperson for Bembridge RNLI said: "Members of the public called this in last night and potentially saved lives.
"This is what we need. Be vigilant, be aware and call 999 and ask for coastguard if you are ever concerned."