there are a few i can think of.
1st. closest to the chain ferry. on coming in to the harbour go to the right/east of brownsea island, hug the coastline about 150yds out, approx 600yds, you are looking for a floating pontoon. (not always open)
2nd is the the left of the lifting bridge by poole harbour wall, opposite the police & harbour authority boats
3rd. cobbs marina, which is past the harbour wall at poole, under the lifting bridge & passed the lifeboat college.