I was just out there this past weekend paddling. Don't know too much about paddling in the fresh, but as for salt:
The Porcupine Islands right out of Bar Harbor would be a mellow paddle. There are 4 of them and you can pick which ones to go around for a loop. One of the islands, Bar Island, can be walked to during low tide. This area is pretty protected from the ocean, so you don't get a lot of waves.
Another nice paddle would be paddling North from Southwest Harbor into Somes Sound (the only Fjord in the lower 48 US) - you could also get into Somes from the northern end, at Somes Harbor but the walls are steeper/cooler near the mouth of the sound so I'd probably prefer starting from SW Harbor.
Another more adventurous paddle would be leaving out of Seal Harbor and going to Little Cranberry Island. It's about a 3 mile paddle each way, but timing it with the tides it's not that bad.
When planning any paddles in the ocean, try to time your coming/going with the tides - it'll make your paddle a lot easier and faster.
Have a good time!!