Boats on Derwentwater are a bit more convenient than those on Ullswater. They're much more frequent and stop at many more places.
From Keswick you can take a boat to any of about 8 landings around Derwentwater. Boats leave every 15 minutes or so and alternate between heading clockwise and anticlockwise around the lake.
For those used to the boats on Swiss lakes the price of short journeys on Derwentwater will be something of a shock! It's a nice way to see the lake, but an expensive mode of transport!
A nice idea is to take a boat to one of the further landings (like Lodore), walk part way round the lake to say Brandlehow or Nicol End and take the boat back the rest of the way. This way you can miss out the "boring" parts of the walk (the bits away from the lake).
There's quite a good bus service around Derwentwater. From Keswick there's a bus that goes down the length of Borrowdale to Seatoller, ones to Penrith, Workington, Cockermouth, Glenridding and Buttermere.
On Sunday the National Trust run a bus up to Watendlath, which is good for those who can't manage the walk or who just want to get a head-start.
The best bus though is the one that runs right through lakeland from Keswick to Windermere via Thirlmere, Grasmere, Rydal, Ambleside and on to Kendal and Lancaster - quite an epic journey!
The timetable varies with day and season, so check online or at the bus station. Most (but not all) of the services are operated by Stagecoach.