As for what to see the 2:
There is a fueling barge just of the main channel towards, Brownsea Island, I have used it but it can be a difficult to get on and off as it quite high.
They is also fuel at Satlerns and Cobbs Quay, (Ring Cobbs as they do run out sometimes).
Definitely worth following the main channel as far as the ferry docks then turn right and heading pass the old town quay. Plenty of kiosks selling food. If Dolphin marina are not bush they sometime let you moor up for a few minutes, to get to the town quay. If they direct you round the back think twice if your rib is large as it is quiet narrow.
Also worth following the main channel up the harbour, towards the SBS base, just before it is a small marina Lake Yard, you can get good food here, but I am not sure if you have to be a member.
It is possible to get all the way to Wareham, given a bit of tide. Worth head up to the top of the harbour as it has large nature reserve as is quite beautiful and quiet.
You can also get right around Brownsea Island again with a little tide.
It is world getting a chart !