Careful, it's easy to run aground in Poole Harbour.
You can go most places on a high tide, but if it's getting low you will need to navigate carefully - or you will be out of the boat, dragging it across the mud!
To that end, make sure you wear a wetsuit or some kind of dry suit if you are going to explore the nether reaches of the harbour.
You should be OK if you stay within the channels though.
There are a lot of private islands in the harbour, so be mindful of that if you are thinking of beaching anywhere.
If you are heading towards the entrance, don't be surprised to find some very large continental ferries bearing down on you all of a sudden (as well as all sorts of other weird and wonderful craft). Don't be intimidated by them, just give them a wide berth and you'll be fine.
Apart from that, it is a fantastic place to explore in a boat - enjoy!