Derwent Water is many people's favourite Lake District lake. It certainly has much to recommend it, with Keswick and Skiddaw at its northern end, Borrowdale at the south and dramatic mountains to both sides it had one of the most varied surroundings of the main lakes.
I can thoroughly recommend using the Derwent Water ferries to see this lake. The ferry stops at seven landing stages spread around the entire lake, and runs in both directions, allowing for a great deal of flexibility, although I wouldn't recommend relying on the last ferry of the day to get back to the opposite end of the lake - if its full then you will have a long walk coming up!
Just above the eastern shore of the lake, you will find one of the most famous viewpoints in the entire Lake District - Ashness Bridge. Take the road to Watendlath (itself worth a visit). You will actually drive over the bridge (the car park is just beyond it). The view of Skiddaw across Ashness Bridge is deservedly famous.